For the VSC-HVDC damper converter valve system, the voltage-balance characteristics of the charging are studied. The installation project of Zhoushan VSC-HVDC bridge arm damping recovery devices is taken as the background. Based on the design principle of the bridge arm damping converter valve, a sub-module mathematical model is constructed which can reflect the sub-module power supply and board power consumption. Through the theoretical analysis of the voltage-unbalance mechanism and the influencing factors of the sub-modules in the charging process of the damped converter valve, the corresponding solutions are put forward. The correctness of the theoretical analysis and the effectiveness of the solution are verified by simulation and test results, which provide technical support for the smooth implementation of the installation project of Zhoushan VSC-HVDC bridge arm damping recovery devices.