To improve the performance of servo system, online suppression of mechanical resonance based on adapting notch filter is adopted. The expansion of bandwidth in servo system will exceed the natural mechanical resonance frequency. The originally ignored influence of elastic components will be more obvious, and mechanical resonance is caused. Firstly, the theoretical analysis of 2-mass model with elasticity load is carried out. The results of simulation verify this resonance mechanism and phenomenon. By FFT method to real-time analyze the electromagnetic torque current, resonance characteristic can be identified online and quickly. Based on identification result of resonance characteristic, parameters of notch filter can be online-set automatically. It can suppress mechanical resonance quickly and effectively. Experiment results show this method can online-suppress the resonance caused by elastic load automatically. Resonance system with high stiffness sustained oscillates at natural torsional frequency when the system is discretized. This method which uses adapting filter to suppress mechanical resonance doesn’t need to change the parameters of controller while high stiffness of control is satisfied.