Abstract: Short circuit fault is one of the main issues affecting the safe operation of flexible DC transmission systems. This paper focuses on the pole-to-pole DC fault. Firstly, a hybrid MMC structure based on series double capacitor submodule（SDSM） and half bridge submodule（HBSM） is studied. Compared with traditional half bridge full bridge submodule hybrid MMC, this structure has lower cost and loss. The configuration ratio design requirements of SDSM and HBSM under transient operation of DC side short circuit are analyzed; Secondly, considering the lack of autonomous negative level output capability of SDSM, a fault crossing strategy based on current phase is proposed. The idea of hierarchical control is adopted to construct a phase current state matrix and combine it with its phase difference to ensure that the converter station can operate with reduced voltage during faults and maintain stable system operation; Finally, the above analysis and the feasibility of the proposed fault traversal strategy were verified through simulation in Matlab/Simulink.