In this paper, we design a novel knee exoskeleton. The designed exoskeleton has a 1 DOF with knee joint actuated in the sagittal plane. The exoskeleton is actuated base on Series Elastic Actuator (SEA) instead of traditonal stiff actuator. SEA is a compliant actuator, having the advantages of low output impedance, low friction, high quality force control and backdrivability. Thanks to these nice characteristics, it is easy to realize a safe and friendly human-machine interaction compare with traditonal stiff actuator. In order to achieve the goal of assistance as needed, a robust force control must be guaranteed. Sliding-Mode Control (SMC) is a way to robust force control. Instead of using the standard SMC, we employ Integral Sliding-Mode Control Algorithm (ISM) to avoid reaching phase, where the performance of system is not so good. Meanwhile, we also use continuous approximation of SMC, which can eliminate the effect of chattering. To verify the efficiency of ISM, we excutes the torque tarcking experiments. We also use the classical PID algorithm to compare with ISM. From the results of experiments, ISM outperform much better than PID, tracking the torque trajectory with high accuracy whereas employing PID produce high errors.