Introduces the significance in UHVDC transmission line protection and control principle of the open line test, Analysis the influences of the variation of firing angle of the converter on open line voltage, when the firing angle of the converter is reduced to less than 60°, the DC voltage can be still risen to its rated value, and depends on the structural parameters of DC transmission line and the intensity of the corona discharge. In the commissioning test of Tianshan to Zhongzhou UHVDC project approve these conclusions. When the firing angle of the converter is reduced to 45.1°, the open line voltage can be risen to rated value 800kV. At the same time, in the practical engineering test, get the current protection principle in the belt line open circuit test can only use direct current and differential current more than 0.025pu as the main criterion of the conclusion.