Abstract: In order to solve the problems of flexible DC blocking caused by the failure of bypass switch of sub-module in flexible converter stations, the action principle and force analysis of bypass switch action process are carried out. Next, from three aspects, that is, on-site inspection, fault recording and the recurrence test of bypass switch closing fault, the fault causes for bypass failure event of a flexible DC sub-module is analyzed in depth.And then, an improved design strategy for bypass switch rejection based on auxiliary voltage criteria is proposed for the non-turning thyristor module. At the last, the effectiveness and reliability of the improved strategy is verified through experiments under four different operating conditions, which avoids the sub-module cracking and converter valve blocking, and further improves the reliability of sub-module operation in flexible HVDC engineering in operation.