The modular multilevel converter(MMC) has emerged as the most attractive conversion technology for high-voltage dc(HVDC) transmission systems. The startup process of MMC is essential for normal operation. The startup process of HBSM-MMC and FBSM-MMC is analyzed in this paper. Due to the different conduction paths, the capacitor voltage of HBSM and FBSM are different in the uncontrolled pre-charging stage. Then, it is explained that the reference voltage of FBSM cannot be tracked during controlled AC stratup process. Furthermore, an improved AC-precharging strategy for FBSM-MMC is proposed, which DC-component and AC-component are controlled separately. Finally, the simulation and experiment are performed to verify the effectiveness of the proposed pre-charging strategy.