The characteristics of the pole-to-pole fault in the tree DC system are complicated, and the transient analytic expressions of hige-order current and voltage can make the calculation difficulty increase significantly. In response to this phenomenon, On the one hand, when the parameters is the same, the impact of the changes of the number about the terminals on the waveform is considered. On the other hand, when the number of terminals is fixed, the influence of the change of the parameter on the waveform is studied. Based on the analysis of single - ended DC fault, the fault calculation method of tree DC system taking into account the interaction between different ports is proposed. This method can reduce the computational complexity of complex DC power network. The calculation results are verified by MATLAB / Simulink simulation tool. The calculated voltage and current values are in good agreement with the results of transient simulation, which provide the theoretical basis for the fault location and calculation of setting value.