The invention discloses a control method for energy storage combined with a thermal power generating unit to participate in AGC frequency modulation. The method comprises the steps of: determining whether an energy storage system is required to act or not; if yes, determining a dynamic model of thermal power generating unit output and energy storage output according to a state space theory to establish a state space equation; determining a target function through the state-space equation according to the frequency modulation effect and the energy management effect of the energy storage system;and in the process of solving the target function, constructing an optimization model by taking the thermal power generating unit output and the energy storage output as constraint conditions, and performing real-time optimization on the energy storage system output by using the optimization model. The problems are solved that in the process that an existing energy storage auxiliary thermal powergenerating unit participates in AGC frequency modulation, an energy storage system adopts a full-power compensation strategy, and energy management is lacked.