The invention provides a control method and system for a photovoltaic power station participating in system frequency modulation, wherein the method comprises the following steps: calculating an active power regulation amount of a photovoltaic power station according to grid-connected point frequency and a frequency change rate of a photovoltaic power station; correcting the initial command valueof active power of photovoltaic power station according to the active power regulation quantity of photovoltaic power station to obtain the final reference value of active power of photovoltaic powerstation; and based on the final active power reference value of the photovoltaic power station, controlling the active power of the grid-connected point of the photovoltaic power station. The technical scheme provided by the invention judges the disturbance magnitude of the power network through the frequency change rate, and adopts the emergency control to immediately increase or decrease different fixed active powers to regulate the active powers of the photovoltaic power station, thereby strengthening the support strength of the power network frequency, reducing the frequency fluctuation amplitude of the power network and increasing the stability of the power network.