Aiming at the problem that the trajectory tracking error of flexible lower limb exoskeleton robot is too large under the condition of external disturbance and parameters uncertainty, a composite position control method based on second-order sliding mode was proposed. Firstly, the flexible lower limb exoskeleton robot is modeled by Lagrange function. Secondly, considering that the system is affected not only by matched disturbance but also by unmatched disturbance, two finite time state observers are used to observe and compensate the two disturbances in real time. In the position control part, the super twisting algorithm is used to ensure the trajectory tracking error of knee joint converging to zero in finite time. Eventually, the stability of the proposed control strategy is proved by the Lyapunov function. The experimental results show that the proposed control strategy has more accurate trajectory tracking effect and robustness than the traditional PD control and sliding mode control, which indicates the superiority of the proposed control strategy.