Sliding mode control strategies have been widely used in controlling robots and have proven their effectiveness. However, since the sliding variable cannot reach zero completely, the tracking error of the system may only converge to a bounded region, and there is also the phenomenon of chattering. In this paper, a fractional-order sliding mode (FOSM) control strategy is proposed for the lower limb rehabilitation exoskeleton robot (LLRER), which improves the dynamic characteristics of the states on the sliding surface through the design of fractional-order sliding surface (FOSS), featuring fast response, small overshoot, and effective suppression of chattering phenomenon. For comparisons, a classical PID controller and a conventional sliding mode (CSM) controller have also been designed. Finally, we built the executable control algorithm on the MATLAB-Simulink platform and conducted experiments on LLRER. The experimental results show that the proposed FOSM controller has higher tracking accuracy and stronger robustness than the controllers used for comparison.