| In order to realize the bi-polar short-circuit fault clear of the flexible DC system, many sub-module topologies with fault blocking capability have been proposed, such as the clamped double sub-module (CDSM). Because the new sub-module structure is different from the half-bridge sub-module (HBSM), its modulation strategy is not fully applicable. Based on the traditional recent level modulation strategy (NLM), this paper proposes an NLM strategy for CDSM. There are two kinds of output UC level settings for this topology, and an improved NLM modulation strategy is designed., can effectively avoid the power loss unevenness caused by different device conduction probability. Finally, taking CDSM topology as an example, the CDSM-MMC simulation model is used in PSCAD/EMTDC software to verify the correctness of the proposed modulation strategy.