When there is fault in submodule of modular multilevel converter (MMC), MMC will run in the state of asymmetric bridge arm,which can cause circular current increasing of fundamental frequency, further affects the performance of MMC. Therefore, this paper considers the comprehensive effect of redundant sub-modules and bridge-arm asymmetric operation, and analyzes the mathematical model of MMC and sub-module (SM) switch function and circular current in detail. On this basis, this paper proposes an optimized redundant protection strategy. Unlike previous strategies, the sub-modules in healthy arms need not be bypassed when SM faults occur and asymmetry degree will be reduced by regulating the base voltage of SM. A model of two-terminal MMC-HVDC system is built in PSCAD/EMTDC, the simulation result proves that the proposed strategy can ensure the system operate well under arm-asymmetric conditions and the ability of the converter to resist the sub module fault is enhanced.