Fault characteristics in offshore wind power low-frequency transmission lines are significantly affected by the control strategies of power converters on both ends, making fault behavior and its impact on traditional protection schemes complex. Based on the control strategy of the converters at the two ends and the fault composite sequence network, this paper theoretically analyzes the applicability of conventional current differential protection and concludes that there is a risk of malfunction or failure to operate. Based on the characteristics of voltage and current waveforms after faults, a protection method based on the waveform similarity of the positive-sequence differential components is proposed. This method first extracts the fault components of current and voltage at the protection installation points on both sides of the line. The symmetrical component method is then used to obtain the positive-sequence components of the differential current and voltage, and the positive sequence differential voltage is corrected accordingly. Then, an improved cosine similarity measure is used to calculate the correlation between the positive sequence components of the differential current and the corrected differential voltage, while the difference in correlation is used to distinguish between internal and external faults. Finally, a PSCAD offshore low-frequency transmission system model is built, and fault scenarios with various fault locations, types, transition resistances, and sampling frequencies are analyzed. The results verify the speed and effectiveness of the proposed protection method.