Abstract: In recent years, the rapid development of renewable energy has brought challenges to the stability of the power grid, and put forward higher requirements for the frequency modulation capability of conventional coal-fired units. However, due to the large thermal inertia of the large-capacity coal-fired unit, the excessively fast frequency modulation speed will seriously damage the safety and economy of the coal-fired unit operation, and the use of flywheel energy storage system to assist the coal-fired unit to adjust the frequency can solve the above problems. Based on the working characteristics of coal-fired units in power generation, the mathematical model of boiler system and steam turbine system was constructed. At the same time, according to the working principle of permanent magnet synchronous motor, the control strategy of flywheel energy storage and discharge process was studied. And we simulated the process of the secondary frequency modulation in the regional power grid which the flywheel energy storage system and the conventional thermal power unit participated in. The simulation model was built in MATLAB/simulink to verified the proposed control strategy and obtained the simulation results: the coupled flywheel energy storage system can reduce the system frequency variation by about 1/2, while reducing the fluctuation of output power and the boiler’s main steam pressure. The results show that using the flywheel energy storage system to assist the coal-fired unit to modulate frequency can not only greatly improve the quality of frequency modulation, but also reduce the fluctuation of output power and the boiler’s main steam pressure, then prolong the life of the unit.