The invention provides a battery energy storage power station power control method used for frequency modulation and a battery energy storage power station power control system used for the frequency modulation. The battery energy storage power station power control method used for the frequency modulation comprises the steps of (A) reading a frequency-modulation active power required value of a battery energy storage power station and relative data during working of the battery energy storage power station in a real-time mode, storing and managing the active power required value and the data, (B) calculating power command values to be assigned to various battery energy storage units in the battery energy storage power station, (C) summarizing the power command values of the various battery energy storage units, and outputting the power command values to the battery energy storage power station. The battery energy storage power station power control system used for the frequency modulation comprises a communication module, a data storage and management module and a frequency-modulation power assignment control module. The battery energy storage power station power control method used for the frequency modulation is achieved through an industrial personal computer and a communication platform. According to the battery energy storage power station power control method used for the frequency modulation and the battery energy storage power station power control system used for the frequency modulation, not only can the function that frequency-modulation corresponding to an automatic generation control (AGC) system is supported by the battery energy storage power station be achieved, but also the aim that real-time power of a megawatt level lithium ion battery energy storage power station is effectively controlled and assigned is achieved.