A Thevenin equivalent circuit model with variable parameters is proposed for the dynamic change of internal system parameters with environment factors and state-of-charge (SOC) when power battery packs are running. Starting from the fact that the state of charge is the main factor affecting the parameters of lithium batteries, the traditional Thevenin model is used to simulate the invariant parameters of each module, and the proposed model is represented by the dynamic variables varying with SOC. HPPC gets the model parameters, and the simulated terminal voltage of the battery is basically consistent with the actual terminal voltage waveform, and the trend is consistent. The experimental results show that the model can effectively reflect the working characteristics of the battery under normal working conditions and has good accuracy.