There are many capacitive devices in the power system, which play an important role in the substation. The insulation failure not only affects the safe operation of the whole substation, but also endangers the safety of other equipment and people, so it is of great significance to realize the safe and stable operation of capacitive equipment. In this paper, the expression method of dielectric loss of capacitive test is calculated based on the basic principle of circuit, and the typical dielectric loss is measured by using oscillating wave and digital bridge, and it is found that the damping oscillation voltage can be accurately measured. Compared with the sine wave voltage experiment, it is found that the measured value of damping oscillation voltage experiment is always smaller than that of sine wave experiment, because the dielectric loss of typical test is mainly caused by the conductivity loss. The damping oscillation voltage gradually attenuates and the conductivity loss gradually decreases, so that the measured value of dielectric loss is smaller than that of the sine wave experiment.