Abstract: FHMMC with symmetrical bipolar and dual converter valve groups in series are adopted at two receiving-end converter stations of ±800 kV Kun-Liu-Long VSC-UHVDC project. Under different charging conditions, due to the different charging characteristics of FBSM and HBSM and their modular series branches, the capacitance voltage of sub-modules is difficult to balance, resulting in the failure of start-up and deblocking for converters. In this paper, the start-up and charging strategy for series FHMMC converter valve groups under conditions of short-circuit and non short-circuit at DC sides is proposed. FBSM T₃, T₄ and HBSM T₂ are triggered for different conditions, respectively, and charging path and object are forcibly converted to realize the balance charging for capacitance. A pole charging sequence control strategy is proposed to reduce the negative voltage of the second converter valve group which is charged by the first converter valve group through the stray circuit. The start-up and charging strategy is verified by field tests and can be used in engineering practice.