Aiming at the retardation and bounded perturbation phenomenon of traditional pneumatic muscle knee joint, the dynamic equation of flexible knee joint of exoskeleton pneumatic muscle is constructed, and a Higher Order Sliding Mode Observer based Sliding Mode Control (STC-HOSMO) based on super-twist control algorithm is proposed. The algorithm realizes the motion control of the flexible knee joint by mapping the dynamic parameters of the flexible knee joint and the pneumatic artificial muscle. In order to verify the feasibility of the algorithm, a Matlab-Simulink simulation model and an experimental platform for flexible knee joint were built, and the simulation calculation and test data showed that the designed STC-HOSMO controller can control the joint with high motion accuracy and robustness under a large range of motion, and can effectively inhibit the traditional pneumatic artificial muscle robot joints that are prone to large jitter, hysteresis and bounded disturbances, showing high compliance. This paper provides a theoretical basis and experimental data for the application and development of flexible knee joint in exoskeleton R&D and design.