By modeling the threephase inverter circuit and reducing the model order, a secondorder power supply model with power electronic interfaces is proposed for the lowfrequency oscillation analysis. Firstly, the eigenvalue analysis is performed in a fourmachine tworegion system including power electronic interface power supply. Then, the results of the analysis are compared with those of higherorder models to verify the validity of the proposed model. In addition, based on this model, this paper also analyzes the influence of power electronic interface power supply capacity on the oscillation frequency and damping ratio of interval and local oscillation. The analysis results show that as the capacity of power electronic interface power supply increases, the oscillation frequency of interval oscillation and local oscillation would increase. The damping ratio also increases. Finally, based on the reducedorder model, the droop controller gain for the power supply of power electronic interface is adjusted, and the higherorder model of the power electronic interface power supply is used to simulate the setting parameters. The simulation results further verify the validity of the reduction order model proposed in this paper.