Abstract: In order to solve the problem of insufficient frequency modulation of thermal power units caused by large-scale new energy power grid connection, we proposed an optimal control strategy for thermal power units to maintain power grid frequency stability assisted by energy storage based on the characteristics of fast response of energy storage batteries. By analyzing energy storage dead zone and thermal power unit dead zone, the frequency deviation is divided into three regions with these two dead zones as the boundary. According to the frequency modulation requirements of each region and the advantages of virtual droop control and virtual inertia control, we adopted different strategies to suppress the deterioration of frequency deviation and its change rate to the greatest extent. Considering the energy storage capacity, we adopted the optimized logistic function to adjust the unit power coefficient（i.e., droop and inertia coefficients） to prevent SOC from reaching the off-limit region too quickly. Finally, we used Matlab/Simulink to simulate the step load disturbance and continuous load disturbance. The results show that the control strategy can not only reduce the output power of thermal power unit, but also significantly suppress the deterioration rate of frequency deviation, reduce the time of frequency stability and improve the stability of the system.