Abstract: Polyimide（PI） films are widely used in the insulation of pulse width modulation（PWM）inverter motors with its excellent dielectric properties. Nano filler is great helpful to improve the insulation property of PI. A PI film with the nano alumina（60nm） content was made for researching the PI film’s surface discharge property under the square impulses. The content of alumina was 1%,2%,5%,7%,10%. Under the different temperatures the number of partial discharge and the corona-resistance of PI/Al₂O₃ films were tested. The microstructure of the PI/ Al₂O₃ film before and after discharge was observed by scanning electron microscope. The chemical bonds of pure PI and PI/ Al₂O₃ were also tested by Fourier transform infrared spectroscopy. The results show that, the nano Al₂O₃ is diffusely distributed in the PI film. Compared with pure PI films, the characteristic absorption peaks of PI/Al₂O₃ is reinforced. After 4 h aging, the C-O-C and C-N-C are disappearing. Other absorption peaks are weakening. Nano Al₂O₃ fillers are improving the corona-resistance of the composite film and are reducing the number of partial discharge. The number of partial discharge is decreasing with the increasing of temperature. Chemical bond rupture is the main reason of degradation of the composite film under the surface discharge. The interface from doping of the nano particles and its good heat-conducting can improve the insulation property of PI films.