<p>For offshore wind power low-frequency transmission system fault, the fault current on both sides of the line by the influence of the control strategy of the amplitude is limited, the phase angle of the open swing increases, resulting in the failure of the traditional longitudinal differential protection criterion. In order to solve this problem, this paper proposes a longitudinal protection method for offshore wind power low-frequency transmission lines based on the absolute value difference of the imaginary part of the integrated impedance. The integrated impedance is defined as the ratio of differential voltage and differential current, and the corresponding protection criterion is proposed according to the characteristics of the integrated impedance during faults inside and outside the zone. By comparing the absolute value of the imaginary part with the integrating value, the in-zone and out-of-zone faults of low-frequency transmission lines can be recognised. Finally, the simulation model of offshore wind power low-frequency transmission system is constructed by using the PSCAD simulation platform, and the reliability and effectiveness of the proposed protection method is verified by processing the fault data through MATLAB. The simulation results show that the protection method is theoretically almost independent of the control strategy at both ends, can be operated reliably and according to phase when different fault types occur at different locations of the line, and has a strong ability to withstand transition resistance.</p>