The short-circuit current amplitude of photovoltaic power supply is limited and the phase angle depends on the control system. It is affected by the tolerance ability of current and response characteristics. As a result, the performance of ratio differential protection is reduced, and the transmission line of a photovoltaic station lacks an effective main protection principle. In order to solve this problem, based on the analysis of the fault characteristics of photovoltaic stations, Tanimoto similarity is introduced to describe the waveform difference between the short-circuit current of photovoltaic stations and a synchronous system. After analyzing the change of similarity coefficient during internal faults and external faults, a novel protection based on Tanimoto similarity is proposed. The simulation results show that the method can correctly reflect the internal faults and external faults. It is suitable for the weak output of a photovoltaic station and has good identification ability for high fault resistance. This work is supported by the Science and Technology Project of State Grid Corporation of China (No. SGNW0000DKJS1900197).