The invention provides a MMC flexible DC sub-module and a MMC flexible DC converter valve, and relates to the technical field of the flexible DC power transmission. The MMC flexible DC sub-module comprises two capacitors in parallel, each capacitor comprises a capacitor core in a capacitor shell, and insulating cement is poured into the capacitor core and the capacitor shell; and the capacitor core is composed of multiple capacitor elements in series or parallel. Through two parallel capacitors, the volume, the weight and the cost of the MMC flexible DC converter valve with the same capacity level are reduced, and the anti-seismic performance and the capacitor running reliability are improved.