Ground fault tests of ultra-high voltage direct current (UHVDC) transmission line indicate that voltage differential protection of the other pole may be false tripped when the ground fault happens on the line of the pole with a larger number of operating valve groups. In order to solve this problem,it is pointed out that the transmission velocity of common-mode of the fault traveling wave is less than that of differential-mode,and there is a time difference between common-mode and differential-mode to reach the protection measurements. Then,it is analyzed that the cause of the voltage differential protection false tripping is that the time difference between common-mode and differential-mode to reach the protection measurements is longer than the setting time of the protection. In addition,the polarity of common-mode component is used as the criterion to identify the fault pole and the protection of voltage mutation is improved accordingly. Finally,a real-time digital simulation model of UHVDC transmission is established to verify the correctness of the analysis results for the impoved method.