A photovoltaic power generation system is controlled by an inverter vector, and its short-circuit transient characteristics are different from those of the traditional power supply. When the power grid fails, its output current may contain the second harmonic, and the influence of these second harmonics when connected to the weak grid cannot be ignored. Taking the symmetrical fault condition of a photovoltaic grid-connected power generation system as an example, this paper first analyzes the phenomenon of single-frequency input dual-frequency output of a three-phase grid-connected inverter based on phase-locked loop, and secondly considers grid-connected control and system low-voltage crossing. Based on the influence of policy switching, the mechanism of current second harmonic generation in the fault transient process is analyzed and the second harmonic analysis expression is derived. It is pointed out that the second harmonic generated by photovoltaic grid-connected system may threaten the reliability of transformer differential protection when the fault occurs in the transformer zone. Finally, based on the correctness of PSCAD/EMTDC simulation verification theory, it provides a reference for the adaptive analysis of grid relay protection after the subsequent PV system. This work is supported by Key Project of Smart Grid Technologies and Equipment of National Key Research and Development Program of China (No. 2016YFB0900600) and Science and Technology Project of Chinese Academy of Electric Power (No. JBB51201901542) (Alibaba-Tibet-China Networking Project Protection Adaptability Analysis).