The invention relates to a flexible DC system starting method and device based on a modular multi-level current converter. The device comprises the following steps: S1, after the current converter station uncontrolled charging is finished, newly adding a sub-module for conducting a T2 tube for each bridge arm, thereby gradually adding the number of the submodule of each bypass; charging residual sub-modules to the charging voltage threshold through transformer valve side line voltage, and executing sub-module dynamic voltage-sharing control under a condition of maintaining unchanged total number of the submodule of each bridge arm bypass; S2, when receiving an unlocking instruction, stepping the DC voltage to a rated modulation ratio corresponding numerical value; and S3, after the DC voltage is stabilized, ramping the DC voltage to a DC voltage instruction value, thereby realizing the smooth starting of the flexible DC system. after the uncontrolled charging of the current converter station is finished, the effective voltage-sharing of the current conversion valve submodule at any time slot can be realized, the time for dispatching the unlocking instruction is unlimited, and the AC system impact and heavy harmonic problem caused by low DC voltage at unlocking early stage of the current converter station can be effectively avoided.