In order to realize the bi-polar short-circuit fault clearing of a flexible DC system, many sub-module topologies with a fault blocking capability have been proposed at present, such as the clamped double sub-module (CDSM). Since this novel sub-module structure is different from the half-bridge sub-module (HBSM), its modulation stra-tegy is not fully applicable. Based on the traditional nearest level modulation (NLM) strategy, an NLM strategy for CDSM is proposed. Considering that there are two kinds of output U_C level settings for the CDSM topology, an improved NLM strategy is designed, which can effectively avoid the power loss unevenness caused by different device conduction probabilities. Finally, the CDSM topology is taken as an example, and a simulation model of CDSM-MMC is built using software PSCAD/EMTDC, which verifies the proposed modulation strategy.