The battery energy storage system (BESS) has been widely used in power grids with the advantages of high control accuracy and fast response speed. In order to use the advantages of BESS in primary frequency regulation (PFR) of a power grid, this paper proposes an integrated control strategy based on state of charge (SOC) and frequency deviation. First, the droop coefficient and the virtual inertia coefficient based on SOC are designed to improve battery cycle life. Then the weighting coefficients based on frequency deviation are introduced to combine the droop output with the virtual inertial output. When the frequency deviation is small, the weighting coefficient of the virtual inertia output is increased to stabilize the frequency. When the frequency deviation is large, the weighting coefficient of the droop control is increased to quickly adjust the frequency deviation. When the frequency deviation exceeds a certain limit, BESS performs frequency support during fault ride-through. In addition, when the system is in good condition and the SOC is low or high, the SOC recovery is performed. Secondly, an evaluation index of BESS participating in the PFR of the power grid is proposed to quantitatively evaluate the frequency regulation performance and the SOC maintenance performance of this strategy. Finally, a simulation model of BESS is built in PSCAD/EMTDC, and the frequency regulation and the SOC maintenance performance of this strategy are verified by simulation under step load and random load disturbance, transient short-circuit fault and intermittent output of PV conditions. The simulation results indicate that this strategy can improve the frequency regulation performance and maintain the SOC within a reasonable range. The results can provide a reference for equipment manufacturers of BESS to design control and protection parameters reasonably, and have practical significance for improving BESS's grid-connected performance. This work is supported by the National Natural Science Foundation of China (No. 51607069).