In the true bipolar modular multilevel converter based high voltage direct current (MMC-HVDC) systems, the probability of occurrence of the single-phase-to-ground faults at the valve-side is arguably low, but it may cause serious overvoltage on the direct current (DC) side. The overvoltage characteristics and generation mechanism is studied when a single-phase-to-ground fault occurs on the valve-side of the MMC in HVDC systems. Firstly, the charging and discharging circuits of the sub-module capacitors for each phase are given before and after blocking. The generation mechanism of the overvoltage of the arm and healthy pole line is investigated then. Finally, simulations are performed based on Xiamen two-terminal HVDC grid to verify the accuracy of the mechanism, and the effect of the blocking delay on the overvoltage is simulated and analyzed. The results show that the increase in the voltage of the arm, the healthy pole line and the metallic return line are caused by the charging and discharging of the distributed capacitances in DC lines. Besides, the shorter the blocking time, the lower the overvoltage amplitude.