Abstract: DC line ground fault is one of the most prevalent fault types in high-voltage direct current (HVDC) systems based on modular multilevel converters (MMCs). To satisfy the current breaking requirement of DC circuit breakers, the fault must be cleared prior to MMC blocking. The blocking instant of MMC is directly determined by the capability of insulated gate bipolar transistors (IGBTs) in sub-modules to withstand the fault arm current, making it of great engineering significance to conduct refined analysis on the characteristics of single arm current during fault periods. Firstly, this paper elaborates on the basic working principle of MMCs. For unipolar ground faults in DC lines, the discharge paths of arm capacitors are discussed respectively under two operating conditions, i.e., AC power supply grounded and ungrounded. Subsequently, based on the equivalent circuit model, the analytical formula of the single arm fault current is derived, considering the dynamic variation of capacitance before fault blocking. Finally, a 51-level true bipolar MMC-HVDC two-terminal DC transmission model is established on the RT-LAB simulation platform. Simulation comparisons demonstrate that there exists a current path for the discharge current of the arm fault capacitor on the AC side, and the proposed calculation method for the arm fault current exhibits high accuracy, which can provide theoretical support for the optimization of MMC fault protection and blocking strategies.