Abstract: A design method of internal model control-PID (IMC-PID) controller is proposed for a fractional order system. First, the complex fractional order model is reduced based on the least square principle. Second, the design method is extended to the fractional order system, and an internal model PID controller for fractional order system is designed based on the simplified model. The controller has only one adjustable parameter. Robust tuning of the controller parameter is realized based on the maximum sensitivity specification M_s. The simulation results show the model simplification method is simple and effective. Moreover, the proposed controller based on the simplified model can not only overcome the shortcomings of the parameter tuning of the fractional order PID, but also provide better tracking characteristics, disturbance rejection performance, and robustness.