In offshore wind power flexible DC transmission systems, negative sequence suppression control strategy is adopted on both sides of the power electronic equipment. When a fault occurs on the AC collection line, the phase angle and amplitude of the fault current on both sides change significantly, which reduces the reliability of the traditional power frequency based distance protection. To address this issue, the structure of the offshore wind power AC collection system is described first, and the implementation method of the negative sequence suppression strategy is analyzed. Secondly, the adaptability of traditional distance protection is evaluated by considering the capacitive current, and the cosine value of the phase angle difference between the phase current and its leading phase current under different faults is analyzed. Based on this, the Hankel matrix is used to reflect the mutation degree of the characteristic quantities, and a single terminal protection principle for AC collection lines based on the singular entropy of the cosine value of the phase angle difference is proposed. Finally, an offshore wind power system model is built in PSCAD/EMTDC, and the effectiveness of the proposed scheme is verified. The results show that the proposed method is highly sensitive, reliable, and is capable of withstanding 20 dB noise and 100  transition resistance.