To tune PID parameters according to closed loop response, rather than the information of model parameters, the influence of PID parameters on the closed loop performance is first analyzed. Then a model free PID parameter tuning method is proposed according to the principle of least vibration and maximal open loop gain. This method only requires the information of vibration frequency of the closed loop response curves, thus the influence of model parameter identification error on the tuning result is avoided and the tuning process is simplified. Finally, experimental results show the effectiveness of the proposed method.