A flexible drive knee joint rehabilitation robot comprises a frame, a seat, a left training mechanism and a right training mechanism, the seat is arranged at the bottom of the frame, the left training mechanism and the right training mechanism are located on the left side and the right side of the seat respectively, the left training mechanism comprises a left thigh section, a left shank section and a novel parallel spring type pneumatic artificial muscle driver, the upper end of the left thigh section is fixedly connected with the seat, the lower end of the left thigh section is rotationally connected with the upper end of the left shank section, and two ends of the novel parallel spring type pneumatic artificial muscle driver are rotationally connected with the left thigh section and the left shank section respectively; and the right training mechanism comprises a right thigh section, a right shank section, a flexible cable, a servo driving module and a variable stiffness module, the upper end of the right thigh section is fixedly connected with the seat, the lower end of the right thigh section is rotationally connected with the upper end of the right shank section, one end of the flexible cable is connected with the right shank section, the other end of the flexible cable is connected with the servo driving module, and the variable stiffness module is arranged on the flexible cable.