A large number of photovoltaic systems (PVs) are connected to the power grid, which reduces the power system’s rotational inertia and primary frequency modulation resources, resulting in the increase of frequency fluctuation during grid operation. Therefore, this paper proposes a control strategy for PVs to actively participate in frequency regulation. The control strategy includes active power frequency droop control and virtual inertia control. The value of the coefficient matches the frequency modulation ability of the PVs under time-varying irradiance, and analyses the influence of the parameters of virtual inertia control on the frequency regulation. Finally, a simulation example based on MATLAB/Simulink verifies the effectiveness of the theoretical analysis and the feasibility of the control strategy.