The open line test（OLT） voltage differential protection acted while poleⅠ of the Gezhouba-Nanqiao±500 kV high voltage direct current（HVDC） transmission system was taken OLT on September 28， 2008 at Gezhouba Converter Station. On the basis of the simple introduction of the principle of HVDC system OLT and protection configuration with the practice of Gezhouba-Nanqiao HVDC transmission project， the reason of the OLT fault is completely analyzed. The analysis shows that the equipment fault of control and protection system of HVDC transmission system can lead to related signal（information） sampling and transferring faults， and even lead to DC system outage in severe situations.