After asynchronous interconnection, an ultralow-frequency oscillation of long duration occurred in some isolated systems, which seriously affected the safe and stable operation of the power system. In this article, the damping characteristics of the governor system are analyzed. A proportional–integral–derivative (PID) controller parameter optimization simulation experiment is performed in the actual DC isolated system, and the frequency stability is compared under different PID parameters. A coordinated control method between the speed control system and the DC frequency controller is proposed in this article. Finally, an actual DC isolated system is used as an example to verify the effectiveness of this control method. The simulation results indicate that the time constant of the water hammer effect of the hydraulic turbine in the power grid becomes larger, the damping ratio will decrease, and the frequency stability of the system will deteriorate.