Abstract: Based on PSCAD, the hydropower black start test system model of Inner Mongolia power grid is built. The power flow and dynamic simulation are carried out for the processes of generator charging idle-load line, long line charging idleload transformer, high-power motor startup of thermal power plant and grid connection of thermal power units. Based on the simulation results, the generator self-excitation, switching over-voltage of charging idle-load line, ferroresonance overvoltage of charging idle-load transformer, system voltage and frequency fluctuations caused by starting load are analyzed. Finally, it is concluded that unit 3 of Wanjiazhai Hydropower Station is operating in isolated network mode, which can maintain the frequency stability of the test system under low load conditions, and has strong under - excitation capacity, which can be used as the black start power supply of Inner Mongolia power grid. During black start, self excitation hasn′t happened in the unit, and the bus voltage of each station can not exceed the specified limit. The initial load of thermal power unit shall be cancelled to avoid large fluctuation of system frequency after grid connection.