Single-phase non-isolated photovoltaic (PV) grid-connected inverters may generate leakage current. When the PV proportion of access to the distribution network increases, it may cause a large collecting leakage current in the distribution station area. Excessive collecting leakage current will not only lead to increased harmonics and line heating, but also trigger leakage protection to cause power outages. In order to clarify the generation mechanism of the collecting leakage current and propose a suppression strategy, this paper firstly derives the double Fourier expressions of the total common mode voltage and the collecting leakage current of the multi-grid system, and the dominant factors that affect the collecting leakage current are analyzed based on obtained Fourier expressions. Furthermore, in the PV collection scenario, the grid-connected inverters are grouped according to the switching frequency feature, and the carriers of the inverter group with the same switching frequency are uniformly phase-shifted to suppress the collecting leakage current. Finally, the effectiveness of the proposed suppression strategy is verified by building a simulation model sourced from an actual photovoltaic village.