Large-scale photovoltaic grid-connected power generation causes a power system to face the problem of inertia decline and insufficient frequency modulation capability. It is urgent to actively participate in the grid frequency adjustment of the photovoltaic system. This paper adopts a two-stage photovoltaic topology and proposes a photovoltaic virtual synchronous machine control strategy for active standby tracking. The DC\DC side determines the active standby point of the system according to the maximum power point of the photovoltaic unit operation, so that the system operates in the active standby mode. When the grid frequency is disturbed, the active output of the photovoltaic unit can be adjusted to participate in the primary frequency modulation. The inverter side adopts the droop control scheme equivalent to the synchronous machine, and simulates the high-voltage DC side capacitor as the dynamic response of the virtual rotor response power, and maintains the DC capacitor voltage constant through PI control. A small-signal model of photovoltaic active frequency modulation is established, and the transfer function is used to analyze the influence of DC capacitance parameters on dynamic performance. The simulation analysis under sudden irradiance drop, grid frequency sudden change and different system parameters proves the effectiveness of the proposed scheme. This work is supported by Natural Science Foundation of Hebei Province (No. E2018502134) and Science and Technology Project of State Grid Liaoning Electric Power Co., Ltd. (No. SGLNDK00DWJS1800091).