The fault current in MMC HVDC systems suffers from a quick rising rate and no zero-cross problems, which makes it hard to cut off and has become the main problem when promoting MMC dc systems. Among the different types of faults, the pole-to-pole fault is the most serious one. Hence, to clarify the discharging mechanism during pole-to-pole faults and determine the influential factors, this paper proposes a simplified modeling method for MMC HVDC fault current impact factors analysis. The proposed method uses the simplified MMC circuit when faults happen, and the frequency domain model is then established for fault current analysis. Considering that the fault period concerned always is less than 10 ms, the high-frequency part impedance characteristics of the established model are carefully studied when device parameters are varied. The smoothing reactor has a significant influence on the high-frequency impedance, while the arm inductance and arm resistance have much less impact on high-frequency impedance. This demonstrates, theoretically, that the pole-to-pole fault current is basically influenced by the smoothing reactor rather than other impact factors. The calculation results based on the model also prove that fault current will change a lot if the smoothing reactor’s parameters are different. The analysis verifies the accuracy of the equivalent model and supplies a theoretical analysis tool for MMC HVDC fault current analysis.