The invention discloses an offshore island flexible DC power transmission system island closed loop start method. The method comprises steps that (1), whether an AC power grid area connected with theflexible DC converter station has an AC power source is determined, if not, the progress turns to step (2); step (2), whether a fixed-end DC voltage operation converter station is charged or unlockedis determined, if yes, the progress turns to step 3; step 3, the flexible DC converter station is unlocked in an island mode, a voltage of closed loop control voltage reference waves rises from 0kV toUa kV at the speed of alpha kV/s; step 4, when the voltage rises from Ua kV to a rated voltage UN kV at the speed of beta kV/s, and island closed loop start is accomplished. The method is advantagedin that big disturbance and big impact during island start can be avoided, high frequency components of an output voltage and an output current and distortion at an initial state can be effectively inhibited during zero voltage start, and stable operation of the system is guaranteed.