The AC transmission line is an important part of the grid connection of offshore wind power via MMC-HVDC. When a fault occurs in the line, it presents fault characteristics that significantly differ from the synchronous generator sources, making it difficult for traditional pilot protection to meet the requirements of reliability and sensitivity. Combined with the characteristics of the current variation on both sides of the AC transmission line after a fault, a pilot protection principle based on the similarity of the current variation is proposed. Based on the Kendall algorithm, this paper qualitatively analyzes the correlation between the current variation on the wind farm side and the MMC-HVDC side during internal and external faults in the AC transmission line, and proposes a protection criterion based on the correlation coefficient of the current variation on both sides of the line. A model for MMC-HVDC-connected offshore wind power is built in EMTP-RV software. The validity and feasibility of the new method are verified by simulation analysis of fault type, fault resistance, distributed capacitance, noise, sampling frequency, data window length and control strategy.