A multi-dimensional building set of building block toys capable of being built freely on front and back sides, comprising: a first module, wherein, a front side of the first module has a single first coupling short post (r) or a single column, two columns or multiple columns of first coupling short posts (r), and a back side of the first module has a concave surface, there are multiple columns of convex second coupling short posts (t) in the concave surface and there are a plurality of ribs on sidewalls of the concave surface; the second coupling short posts (t) are distributed as follows: column: the columns are uniformly distributed in the same columns as the first coupling short posts (r) on the front side and located in a middle position between two adjacent columns; line: in the same columns as the first coupling short posts (r) on the front side, each of the second coupling short posts (t) is arranged alternately with the first coupling short post (r) and located in a middle position between two first coupling short posts (r); therefore, in such a column, the number of the second coupling short posts (t) is one less than the number of the first coupling short posts (r); in a column located in a middle position between two adjacent columns of the first coupling short posts (r), each of the second coupling short posts (t) is located in the middle position between two first coupling short posts (r) of the same line; therefore, in such a column, the number of the second coupling short posts (t) is the same as the number of the first coupling short posts (r); a second module, wherein a front side of the second module has a single first coupling short post or a single column, two columns or multiple columns of third coupling short posts (s), the third coupling short post (s) is a cylinder penetrating through the second module; the number of the third coupling short posts (s) may be 1, 2, 3 or any number; a back side of the second module has a concave surface, there are convex fourth coupling short posts (u) in the concave surface and there are a plurality of ribs exactly facing the fourth coupling short posts (u) on the sidewalls of the concave surface; the fourth coupling short posts (u) on the back side of the second module are distributed as follows: column: the columns are located in a middle position between two adjacent columns of the third coupling short posts (s) on the front side; line: each of the fourth coupling short posts (u) is located in a middle position between two third coupling short posts (s); therefore, the number of the fourth coupling short posts (u) is one less than the number of the third coupling short posts (s); and wherein the arrangement being such that both the front side and the back side of the first module can be connected to both the front side and the back side of the second module. column: the columns are uniformly distributed in the same columns as the first coupling short posts (r) on the front side and located in a middle position between two adjacent columns; line: in the same columns as the first coupling short posts (r) on the front side, each of the second coupling short posts (t) is arranged alternately with the first coupling short post (r) and located in a middle position between two first coupling short posts (r); therefore, in such a column, the number of the second coupling short posts (t) is one less than the number of the first coupling short posts (r); in a column located in a middle position between two adjacent columns of the first coupling short posts (r), each of the second coupling short posts (t) is located in the middle position between two first coupling short posts (r) of the same line; therefore, in such a column, the number of the second coupling short posts (t) is the same as the number of the first coupling short posts (r); a second module, wherein a front side of the second module has a single first coupling short post or a single column, two columns or multiple columns of third coupling short posts (s), the third coupling short post (s) is a cylinder penetrating through the second module; the number of the third coupling short posts (s) may be 1, 2, 3 or any number; a back side of the second module has a concave surface, there are convex fourth coupling short posts (u) in the concave surface and there are a plurality of ribs exactly facing the fourth coupling short posts (u) on the sidewalls of the concave surface; the fourth coupling short posts (u) on the back side of the second module are distributed as follows: column: the columns are located in a middle position between two adjacent columns of the third coupling short posts (s) on the front side; line: each of the fourth coupling short posts (u) is located in a middle position between two third coupling short posts (s); therefore, the number of the fourth coupling short posts (u) is one less than the number of the third coupling short posts (s); and wherein the arrangement being such that both the front side and the back side of the first module can be connected to both the front side and the back side of the second module. column: the columns are located in a middle position between two adjacent columns of the third coupling short posts (s) on the front side; line: each of the fourth coupling short posts (u) is located in a middle position between two third coupling short posts (s); therefore, the number of the fourth coupling short posts (u) is one less than the number of the third coupling short posts (s); and wherein the arrangement being such that both the front side and the back side of the first module can be connected to both the front side and the back side of the second module.