Abstract: When an asymmetric fault occurs in the AC collection line of offshore wind power through a flexible DC transmission system, the line has weak fault characteristics due to the negative-sequence suppression strategy adopted by the inverters on both sides, resulting in a decrease in the performance of conventional power frequency AC protection. This paper first analyzes the faulty circuit when an asymmetric fault occurs on the offshore AC collection line, as well as the distribution of negative-sequence current under internal and external faults. It is found that there is a significant difference in the negative-sequence current between the faulty line connected to the same bus and the non-faulty line. Furthermore, by constructing the accumulation of negativesequence current and amplifying the difference in negative-sequence current amplitude between faulty and non-faulty lines, a protection scheme for AC collection lines based on the ratio of transient negative-sequence current accumulation is proposed. The proposed scheme can achieve rapid and accurate identification of fault lines, and has a certain ability to withstand transition resistance. Finally, simulation verification is conducted on the PSCAD/EMTDC platform, and the results show that the proposed scheme is feasible and effective.