Abstract: In a DRU(diode rectifier unit)-based flexible low-frequency AC transmission system of offshore wind power, DRU cannot transmit active power to the offshore wind farm, and the system requires an additional black start power source to provide initial start-up energy for wind turbines. For this reason, a black start strategy using auxiliary converter as black start power source for the above system is proposed. Firstly, the topology structure of DRU-based flexible low-frequency AC transmission system and the grid forming control strategy adopted by wind turbines are introduced. Then, based on the above control strategy, the black start strategy of auxiliary converter and wind turbines are designed. On the basis of the above strategies, the selection method for auxiliary converter capacity is analyzed. Finally, a corresponding simulation model is built on the PSCAD/EMTDC software to simulate the black start process of the system, and all electrical quantities can smoothly reach the normal operation state. The simulation results show that the black start strategy proposed in this paper can realize the smooth start of the DRU-based flexible low-frequency AC transmission system of offshore wind power.