A supporting device for a rail member configured to be mounted to a rack, the rack comprising a first post and a second post, the supporting device comprising: a first supporting frame having a first longitudinal body, a pair of first transverse parts being arranged on the first longitudinal body, each of the first transverse parts having a first supporting feature; a second supporting frame movably attached to the first supporting frame, the second supporting frame having a second longitudinal body, a pair of second transverse parts being arranged on the second longitudinal body, each of the second transverse parts having a second supporting feature; and a third supporting frame movably attached to the second supporting frame, the third supporting frame having a third longitudinal body; wherein one of the first supporting frame and the third supporting frame is fixedly attached to a rail member; wherein the first supporting feature of the first supporting frame is configured to support the second supporting frame, and the second supporting feature of the second supporting frame is configured to support the third supporting frame; whereby when the first supporting frame is fixedly attached to the rail member, a first bracket is arranged on one of the rail member and the first supporting frame, and a second bracket is arranged on one of the second supporting frame and the third supporting frame, each of the first bracket and second bracket comprises a side wall and an end wall perpendicularly connected to the side wall or whereby when the third supporting frame is fixedly attached to the rail member, the first bracket is arranged on one of the rail member and the third supporting frame, and the second bracket is arranged on one of the first supporting frame and the second supporting frame, each of the first bracket and second bracket comprises a side wall and an end wall substantially perpendicularly connected to the side wall, and the supporting device is characterized in that each first supporting feature of the first supporting frame comprises a first supporting section and a first extension section substantially perpendicularly connected to the first supporting section, each first supporting section is substantially perpendicularly extended from the first longitudinal body of the first supporting frame, a first supporting space is defined by the first longitudinal body and the pair of first supporting sections, each first extension section is located outside the first supporting space. a first supporting frame having a first longitudinal body, a pair of first transverse parts being arranged on the first longitudinal body, each of the first transverse parts having a first supporting feature; a second supporting frame movably attached to the first supporting frame, the second supporting frame having a second longitudinal body, a pair of second transverse parts being arranged on the second longitudinal body, each of the second transverse parts having a second supporting feature; and a third supporting frame movably attached to the second supporting frame, the third supporting frame having a third longitudinal body; wherein one of the first supporting frame and the third supporting frame is fixedly attached to a rail member; wherein the first supporting feature of the first supporting frame is configured to support the second supporting frame, and the second supporting feature of the second supporting frame is configured to support the third supporting frame; whereby when the first supporting frame is fixedly attached to the rail member, a first bracket is arranged on one of the rail member and the first supporting frame, and a second bracket is arranged on one of the second supporting frame and the third supporting frame, each of the first bracket and second bracket comprises a side wall and an end wall perpendicularly connected to the side wall or whereby when the third supporting frame is fixedly attached to the rail member, the first bracket is arranged on one of the rail member and the third supporting frame, and the second bracket is arranged on one of the first supporting frame and the second supporting frame, each of the first bracket and second bracket comprises a side wall and an end wall substantially perpendicularly connected to the side wall, and the supporting device is characterized in that each first supporting feature of the first supporting frame comprises a first supporting section and a first extension section substantially perpendicularly connected to the first supporting section, each first supporting section is substantially perpendicularly extended from the first longitudinal body of the first supporting frame, a first supporting space is defined by the first longitudinal body and the pair of first supporting sections, each first extension section is located outside the first supporting space.