Abstract: The system moment of inertia resources is becoming increasingly scarce with the continuous increase of the proportion of renewable energy capacity. It is necessary to accurately estimate it to provide a good basis for the operation analysis. The moment of inertia of a power system is composed of the power generation side one and the load side one. The moment of inertia at the power generation side can be accurately obtained by statistics, while the one at the load side is difficult to obtain due to its large number and dispersion. Starting from the analysis of the load side inertia mechanism, through the analysis of the load power and the voltage change rate before and after the disturbance, the equivalent inertia characteristics of the constant impedance, the synchronous motor and the asynchronous motor load are theoretically deduced in the way of the disturbance method. Then, the inertia characteristics of various loads are clarified and an on-line fast estimation of the overall moment of inertia of the system considering the inertia characteristics of both sides of the source load is proposed. The simulation analysis verifies the correctness of the analysis conclusion of the load side inertia characteristics. Based on the measured data of two frequency fluctuation events in a regional power grid in 2019 and 2020, there is only an error within ±5% between the estimated values of the system inertia by the proposed method and by the disturbance method.