Abstract: The overvoltage poses a huge threat on the power equipment and personal safety. In order to prevent against the overvoltage, by analyzing two kinds of typical operations which are switching on and off no load transformer and bus, the causes of “switching surge” and “resonance overvoltage” are demonstrated. It is proposed that the “switching surge” can be reduced by reducing the no load current, reducing the transformer winding inductance and increasing the transformer parasitic capacitance. The “resonance overvoltage” can be reduced through the switching operation optimization and technical equipment improvement. The preventive measures are suggested through on site example analysis. Practice proves that the hazards in the 220kV substation operation can be completely prevented.