An adjustable structure for rear seat of bicycle, mainly comprising a rear seat, a clamp, and a connection member, wherein: one end of the rear seat is used for carrying an object, the other end thereof opposite to the object has a rear-seat linking portion which is linked to the connection member, one side of the rear seat having the rear-seat linking portion includes a rear-seat linking surface, the rear-seat linking portion is protruded from the rear-seat linking surface, a rear-seat through hole is formed at a center of the rear-seat linking portion, a rear-seat stopping surface is downwardly formed from a top surface of the rear-seat linking portion, and a wall surface of the rear-seat linking portion has a first rear-seat teeth portion and a second rear-seat teeth portion; the clamp is linked to a seat post of a bicycle, one side of the clamp opposite to the seat post has a clamp linking portion which is linked to the connection member, the side of the clamp having the clamp linking portion includes a clamp linking surface, the clamp linking portion is protruded from the clamp linking surface, a clamp through hole is formed at a center of the clamp linking portion, a clamp stopping surface is downwardly formed from a top surface of the clamp linking portion, and a wall surface of the clamp linking portion has a first clamp teeth portion and a second clamp teeth portion; the connection member is used for connect the rear seat and the clamp, two sides of the connection member are respectively arranged a connection portion, a peripheral of the connection portion has a connection surface which is used to attach the rear-seat linking surface and the clamp linking surface, the connection portion is used to fix the rear-seat linking portion and the clamp linking portion, a height of the connection portion is lower than a height of the connection surface, a wall surface of the connection portion has a third teeth portion, the connection portion has a convex body, a wall surface of the convex body has a fourth teeth portion, the third teeth portion of the connection portion adjacent to the rear seat is arranged corresponding to the first rear-seat teeth portion, the third teeth portion of the connection portion adjacent to the clamp is arranged corresponding to the first clamp teeth portion, the fourth teeth portion of the connection portion adjacent to the rear seat is arranged corresponding to the second rear-seat teeth portion, the fourth teeth portion of the connection portion adjacent to the clamp is arranged corresponding to the second clamp teeth portion, the connection portion has a connection through hole, the connection through hole of the connection portion adjacent to the rear seat is arranged corresponding to the rear-seat through hole, the connection through hole of the connection portion adjacent to the clamp is arranged corresponding to the clamp through hole, a top of the convex body of the connection portion adjacent to the rear seat is abutted against the rear-seat stopping surface, a top of the convex body of the connection portion adjacent to the clamp is abutted against the clamp stopping surface, and then the connection member connects with the rear seat and the clamp through multiple screws and nuts; wherein a counterbored hole is arranged at one side of the connection member opposite to the connection portion, and one end of the clamp opposite to the clamp linking portion and one end of the rear seat opposite to the rear-seat linking portion are respectively arranged a hexagonal socket for providing the nuts to fix; wherein the rear seat has at least one sliding slot, and an elastic pushing member is arranged at bottom of the at least one sliding slot so that the object which is guided into the at least one sliding slot is fixed by the elastic pushing member; wherein the clamp has an arc clamping piece for clamping the seat post.