Abstract: Large scale development of offshore wind power will become an important way to achieve the national energy structure adjustment and energy conservation and emission reduction goals. Flexible DC transmission is an important technology for large-scale centralized development of offshore wind power. In order to ensure the high-reliability operation of the off-shore flexible transmission project, a series of tests must be carried out to check the correctness of the converter valve design before it is delivered to the offshore site. For modular multilevel converter with symmetrical monopole topology in an offshore flexible DC transmission project, this paper analyzes the mechanism of abnormal DC partial discharge during dielectric tests on multi-valve unit. Referring to the relevant standards and the experience of previous projects, this paper studies and proposes a test method to solve the abnormal DC partial discharge measurement. On the premise of meeting the standards and specifications, this method can effectively verify the voltage withstand ability and partial discharge level of the multi-valve units with the same topology, and provide reference for the smooth development of dielectric tests on multi-valve unit with higher voltage level in the future.