Abstract: When zero-carbon units represented by clean energy power generation were integrated into the grid on a large scale, frequency fluctuations caused by intermittency and uncertainty became more prominent. Aiming at the problems of insufficient flexibility in frequency modulation and unsatisfactory effect in charge state maintenance of thermal power units assisted by battery energy storage, an energy storage assisted secondary frequency modulation control strategy based on adaptive decomposition of frequency modulation signals was proposed. Firstly, the switching time criterion of regional adjustment demand signal and regional control error signal was given by synthesizing sensitivity, energy storage charge state and frequency deviation state. Considering the climb rate limit of thermal power units and the state limit of stored energy, an adaptive frequency modulation signal decomposition strategy was proposed, and the rule between return gain and state of charge was constructed as the adjustment reference value. Frequency deviation was used to correct the reference value. Finally, the state consistency detection module was used to judge the output of return gain, so as to realize the complementary advantages of the two frequency modulation resources. Effectiveness of the proposed strategy was verified by the simulation of the single-region system frequency response model. Results show that the strategy can improve the flexibility of the system and make each frequency modulation resource form a complementary relationship.