Epoxy resin impregnated paper (RIP) bushings are widely used as transformer bushings and wall bushings. This type of bushing has stable electrical properties, explosion-proof, small size, light weight and other advantages. But its manufacturing process is complicated and difficult, improper process control will result in defects. In order to find out the characteristic parameter of defects, some samples are manufactured to simulate defects and their insulation parameters are investigated in this paper. For example, the dry time is shortened and the moisture is not dried completely during the heating & drying process; crack is resulted by uneven release of thermal stress during solidification process; incomplete degassing of material and residual gas creates bubbles in the condenser core. Insulation parameters such as insulation resistance, dielectric loss factor, and capacitance, frequency-domain dielectric spectroscopy (FDS), polarization and depolarization current (PDC) are studied. Test results indicated that all type of defects may lead to a larger tanδbut a smaller insulation resistance than normal bushing,and different defect bushings exhibit different rates of decrease and stability value at the polarization current. By analyzing FDS curves on different frequency bands and the PDC pattern, different bushing defects can be distinguished effectively. This study can provide reference for factory test and on-site test to diagnose the insulation state of the bushing.