The invention provides a method and a device for an energy storage system for participating in primary frequency control of a power grid. The method comprises the following steps: monitoring a grid frequency and a charged state of the energy storage system; if a grid frequency deviation does not exceed the maximum frequency deviation, monitoring the grid frequency and the charged state of the energy storage system in cycle; if the grid frequency deviation exceeds the maximum frequency deviation, determining that the energy storage system participates in the primary frequency control of the power grid according to the deviation direction of the grid frequency deviation and the charged state of the energy storage system; if primary frequency modulation starting conditions are met, starting a virtual inertial response and primary variable droop control; and adding power adjustment values of the virtual inertial response and the primary variable droop control to obtain a total active power control instruction and giving the instruction to the energy storage system, thereby achieving the primary frequency control. The method can significantly improve the frequency change amplitude and the steady speed in frequency disturbance of the power grid by quick response ability of the energy storage system, and improves the load disturbance resistance of the power grid.