The present invention provides a method for starting a modularized multi-level flexible direct-current transmission converter. The method comprises: connecting a direct-current side pole of each converter station, disconnecting an alternating-current circuit breaker of each converter station, and locking a sub module of each converter station; closing an alternating-current circuit breaker of an active station at one end, and a power supply carrying out passive charging on the active station and the remaining converter stations by using startup resistors; after the passive charging is completed, bypassing the startup resistor of the active station, unlocking the remaining converter stations, and carrying out active charging to reach a rated voltage of a power module; interlocking the remaining converter stations, the converter stations charged by the active station carrying out unlocking and starting direct-current voltage control subjected to a slope until the power module is charged to the rated voltage; and unlocking the remaining converter stations, after entering into a controlled stage again, carrying out grid connection and switching a control policy to a normal operation policy, and increasing power, wherein the converter operates normally. According to the method disclosed by the present invention, voltage consistency of the power module in the starting process can be ensured, so as to avoid startup failure and protection locking caused by unevenness of the capacitance and voltage of the power module.