The essence of double voltage method is that, firstly use the precisely calibrated current comparator bridge and 2:1 voltage divider (In this paper, they are respectively 1 kV inductive voltage divider and auto current comparator 7010B) measure the ratio of two capacitors in the same voltage then turn the voltage value of measured capacitor to be double and test their ratio again. Thus, the exchange capacitance values of the measured capacitor when its voltage rise from one times voltage to the double times can be calculate. The equivalent circuit is shown in Fig.1, and theory derived is follow