Abstract: The system of a joint driven by PMAs(pneumatic muscle actuators) is a strong nonlinear time-varying self-balanced second-order system.Traditional control methods are difficult to overcome the conflict between the control accuracy, the regulating speed and the stability.A second-order active disturbance rejection control(ADRC) is designed,in which a third-order ESO(extended states observer) is adopted to estimate the non-modeling part and external disturbance in real time and to make the linear compensation for system.Meanwhile,the control efficiency is improved by a nonlinear feedback. On the premise of ensuring the stability,the adjusting speed and control accuracy is enhanced.The system's steady-state accuracy is less than 0.1°and the tracking accuracy is less than 0.5°.Even in the case of the unknown or partly known system model,the system using ADRC can still achieve a satisfactory control result.