This paper analyzes the December15，2012accident of pole1emergency switch-off（ESOF）and pole2restarting in Yunnan-Guangdong ultra-high voltage direct current（UHVDC）transmission project.It is pointed out that the earth voltage increase and subsequent damage to the low-voltage valve hall F5arrester caused by the difference of the tap changer positions between the high-voltage and low-voltage valve group transformers，result in the correct operation of87DCM and87CSD of pole1DC protections.The flashover of electrode lines caused by the neutral bus overvoltage during pole1blocking results in the correct operation of electrode lines unbalanced protection（60EL），and then pole2restarts successfully.The risk in the process is thoroughly analyzed，and some effective measures are proposed to reduce the blocking risk of Yunnan-Guangdong UHVDC transmission project.