Wudongde hybrid multi-terminal UHVDC is nowadays the world's highest voltage and largest capacity flexible DC transmission project. It has the ability of non-blocking DC fault ride through. However, the project is a mixed structure of full bridge and half bridge.When the AC circuit breaker of uncontrolled rectifier asynchronous charging is not closed, the full bridge modules of the valve group are charged passively, and the DC port of the valve group presents induced voltage. In some connection modes, the induced voltages are very high, which cause the system to trip. Aiming at the problem of asynchronous charging, the partial voltage characteristics of valve group asynchronous charging from several main connection modes and the influence of induction charging on the system are analyzed. The method of isolating valve group by DC short circuit charging is proposed to solve the problem of valve group partial voltage, and according to the characteristics of asynchronous charging, the method of shortening asynchronous charging time interval is proposed to solve the problem of valve group partial voltage.