Abstract: High-frequency oscillation is frequently occurs in flexible DC distribution systems, due to the influence of the interaction of multiple time scale control links and physical circuits. This paper studies the mechanism of high-frequency oscillation, and establishes a high-order mathematical model of the flexible DC distribution system. Sensitivity analysis of the key parameters affecting the system order has been conducted to realize the model order reduction. The analytical equation of high-frequency oscillation is deduced in time domain considering the influence of system parameters. This paper summarizes the influence law of the DC side filter capacitor, AC side equivalent inductance and controller parameters on high-frequency oscillation. Finally, based on Simulink, the simulation models of low-voltage and medium-voltage flexible DC distribution systems are built, and the simulation results verify the validity of the theoretical analysis.