This paper presents a lower limb rehabilitation robot based on free gait and virtual reality, which is intended to improve therapeutic efficacy. With the help of the robot, patients with motor disabilities can walk with free gait in a virtual environment and choose between passive and active training modes. So the impaired limbs can get appropriate exercise in all clinical stages and the motivation to participate in rehabilitation training is increased. Inspired by this idea, a new free gait generation algorithm is proposed and the working mechanism of the passive/active training mode is explained in detail. And the results of preliminary experiments are presented to demonstrate the effectiveness of the proposed algorithm and the performance of the rehabilitation system.