Abstract: Aiming at the non-linearity as well as the uncertainty of parameters and external interference in pneumatic hydraulic combination control system (PHCC), the function approximation technique (FAT) based the multiple-surface sliding control (MSSC) algorithm is presented in this paper. The Lyapunov method is applied to acquire the stability of closed-loop system and realize the robust impedance control of PHCC system. The impedance control simulation researches are conducted under the 7mm/s repeatedly slope and 0.2Hz sine signal respectively. The simulation results show that the control strategy is able to perform good compliant control with good robustness and the system performs well in both free-space and compliant tracking movements.