In order to study the effect of moisture on PD characteristics for oil-paper insulation in complexelectric field，PD tests with different moistures were carried out under the typical electric-field of oil-paper insulation. The PD characteristics under needle-plane electrodes and plane-plane electrodes were obtained sepratedly in complex electric field with different moistures. The result shows that DC content has a great effect on the inception voltage and PD volume. All the PD volume grow up with the voltage rises under different moisture. In severe non-uniform electric field，PD volume in high moisture is always larger than the one in low moisture or equal to it. In uniform electric field，the PD curve in high moisture crosses with the one in low moisture. Via analyzing， it is concluded that the conductivity in oil is increased by moisture，which makes the space charge disappeared easier from oil-water interface，and then enhances the PD volume. There may be a bubble combination defects PD model in liquid dielectrics，besides a classic bubble defects PD model. The bubble combination maybe the major reason which results in the PD curves in high and low moisture crossing over under plane-plane electrodes.