The flexible joint mechanism with variable stiffness is affected by joint damping when it is controlled by active and passive composite drive, which leads to poor global steady-state of driving regulation. An optimal control design method for flexible joint mechanism with variable stiffness based on compound amplitude saturation nonlinear state feedback is proposed. The dynamic model of flexible joint mechanism with variable stiffness is constructed, and the equivalent stiffness of flexible joint is used. The damping force and the compression of the joint spring are the control constraints. The controlled object model of the flexible joint mechanism with variable stiffness is established. The adaptive damping error is corrected by using the amplitude saturation nonlinear state feedback control method. According to the characteristics of active and passive composite stiffness, the driving inertia parameter of flexible joint mechanism with variable stiffness is adjusted, and the driving regulation control of joint mechanism is realized. The simulation results show that the stability of the flexible joint mechanism with variable stiffness is better and the driving ability is better.