A square-section spring mechanism with adjustable rigidity comprises a pipe body, a spring and a screw, an external thread is arranged on the outer wall of the pipe body, an internal thread is arranged on the inner wall of the pipe body, the screw is screwed into the internal thread to form a first screw pair, and the spring is screwed into the external thread to form a second screw pair. The spring is a cylindrical equal-pitch spiral spring with the rectangular section, the first screw pair and the second screw pair are rectangular screw pairs, the internal thread and the external thread are equal in pitch, the lift angle of the internal thread and the lift angle of the external thread of the pipe body are smaller than the minimum equivalent frictional angle in the first screw pair and the second screw pair, and the rigidity of the spring is continuously adjustable within the design range. The square-section spring mechanism is simple in structure, capable of bearing large loads and high in stability, the rigidity of the spring mechanism can be adjusted under the loading condition, the rigidity adjusting process is simple, the debugging time is greatly saved, and the debugging efficiency of a prototype is improved.