Step PID controller is usually applied in the situation that steady step response is more important than quick response in a DC motor speed control system, but the controller does not work when there exists measurement error. This paper designed a DC motor speed control system based on step-limiting incremental PID controller. The controller limits the increment of the control value of incremental PID by a step to smooth the change of control value, so as to achieve smooth speed shifting. Analytical and experimental results show that owing to the even variation of the control values, this system not only presents steady step response, but also has anti-interference ability in nature.