When multi-terminal flexible DC system occurs pole-to-pole fault, all the conventer station act together, resulting in the fault current increasing rapidly and the fault characteristics are very complex. It is required to remove the fault before the voltage zero crossing point, but also increased the difficulty of solving. Proposing a practical method to solve the current and voltage transient expression. The n-terminal system capacitor discharge process is decomposed into n single-ended capacitor alone process. According to the calculation results, the influence of the parameter change on the fault characteristics is analyzed. The fault model of 3 - terminal ring DC system is built by MATLAB / Simulink software, which verifies the reliability of the method and provides a new idea for the protection setting.