A DC differential protection malfunction accident of pole 1 caused by Yunnan-Guangdong ultra-high voltage direct current（UHVDC）commutation failure is analyzed，which includes commutation failure phenomena，process and equivalent circuit analysis，DC currents on high and low voltage sides and their differential current，and also the malfunction reason for DC differential protection.During commutation failure process，the inconformity of low voltage current transformer characteristic and high voltage current transformer characteristic of pole 1 results in DC differential current between high voltage side and low voltage side increasing，and finally，it results in malfunction of DC differential protection.