The invention provides a frequency modulation method and a frequency modulation device for a power energy storage system. The method comprises the following steps: determining the frequency modulation ability of a power unit; predicting the climb amount of wind power; when the predicted climb amount of wind power is greater than the frequency modulation ability, determining the frequency modulation capacity according to the frequency modulation ability and the climb amount, and using the frequency modulation capacity as a step signal to adjust the power output of the energy storage system; and in the process of adjusting the power output of the energy storage system through the step signal, adjusting the power output quantity of the energy storage system through frequency feedback control. The technical problem that an energy storage device cannot be involved in system frequency modulation effectively under control in the prior art is solved, the steady-state offset of frequency is reduced, and the dynamic response speed of system frequency modulation is increased.