For a person with disability, a functional electrical stimulation (FES) machine is important because it enables them to carry out tasks that they are incapable of performing on their own. In the FES system, one common controller is the proportional-integral-derivative (PID) controller. The literature has proposed a number of complex PID tuning techniques, most of that are based on modern heuristic techniques. The real response, however, is never the same as the theoretical response because of modelling errors and uncertainties, which necessitate constant adjustment of the PID parameters despite their benefits and accuracy. The stability and robustness of the controller may be compromised if the three PID parameters of the FES system are adjusted manually. Hence, this paper focuses with robustness-guaranteed online tuning of the PID controller of an FES system. The technique, known as an indirect design approach, is specifically related to the PID controller’s frequency shifting. In an FES system, the frequency shifting constant and a previously optimized PID controller determine the tuning guidelines for a PID controller. All previous PID parameters will change when the frequency shifting constant is changed, improving the robustness and performance of the control. By comparing the gain and phase margins to the prior optimized PID parameters, the robustness of the suggested PID tuning method was assessed. According to simulation results, the suggested tuning technique significantly increases the PID controller's robustness in the FES system.