The new power system is gradually evolving into a low inertia power system with high penetration of DC and new energy, and the inertia of the load side is gradually increasing. Asynchronous motors occupy a high proportion in the load side, and their frequency support function needs further research. In order to evaluate the equivalent inertia of asynchronous motor under inertial time scale, a small signal model of asynchronous motor is established in electromechanical transient state. The transfer function of asynchronous motor power consumption and system frequency deviation is inferred in this paper. At the same time, the effective inertia of asynchronous motor to power system is inferred and the time-varying characteristics of asynchronous motor to power grid is analyzed. According to the idea of frequency support capability in inertia response stage, the factors which affect inertia frequency response of asynchronous motor are analyzed. The equivalent inertia evaluation model of asynchronous motor is proposed to quantify the frequency support capacity of dynamic load. Finally, a simulation model is built on Matlab/Simulink and PSASP platform to verify the correctness and effectiveness of the evaluation model and method proposed in this paper.