This paper presents a graphical user interface system for a lower limb rehabilitation robot. The mechanism considered for this application is a stationary trainer based on Cartesian parallel manipulator. The trainer is configured with three legs of PRRR (prismatic-revolute-revolute-revolute) configuration connected to the end-effector. The end-effector of the mechanism is a footplate to which the subject’s foot can be attached for therapeutic procedures. The interactive simulator is designed primarily for carrying out the therapeutic procedures of PROM (passive range of motion) of the lower limb. Specifically, the application developed can demonstrate the mechanism motion for hip abduction–adduction, hip flexion–extension, knee flexion–extension and gait training. In addition to these options, the simulator also enables the user to vary the subject parameters like thigh and crus lengths. The proposed interactive simulator design is demonstrated and verified using computer-based numerical simulations. © 2023, The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd.