In order to better tune the PID controller, it is necessary to obtain accurate model information of the controlled object. Due to the low calculation accuracy of ZN setting rules in relay feedback and uncertain characteristic parameters of the controlled object in engineering, it is impossible to use advanced control scheme to accurately control the system. The PID controller parameter selftuning method based on relay feedback is proposed, and the simulation analysis is carried out for the industrial object which is approximately the first order inertial pure lag link. The critical information of the system is obtained through relay feedback. Then the function relationship between the critical information and the model parameters is deduced. Finally, the characteristic parameters are calculated. The experimental results prove that the function relation can identify the inertia time and the lag time in condition of low error, which provides a quantitative reference for the design of PID controller.