In order to isolate the DC magnetic bias current, the capacitive DC current blocking device has been installed in the neutral point of converter transformers. Based on the protective action of capacitive DC current blocking device of the ±500kV HVDC project, the occurring process of the accident, the action principle of the capacitive DC current blocking device are introduced. The reason of the accident is analyzed. The analysis results show that the protection configuration of the DC blocking device is unreasonable. When the DC bias current is large, the thyristor circuit for capacitor protection is continuously turned on, which causes the device to lose DC current isolation effect. At last, the preventive measures are presented.