Polyimide（PI）film is widely used in the insulation of PWM inverter motor. The partial discharge under the square impulse is one of the main causes in insulation failure of inverter fed motors. In this paper

PI film with nano silica（30 nm） content has been made for the research of PI film’s aging under the square impulses and PI film’s action under the nano particle. The micro-morphology and structure of nano silica in PI film is investigated by scanning electron microscopy（SEM）. We can get the life span and withstand voltage strength through the square wave voltage withstand test and corona aging experiment

and explain PI film’s dielectric breakdown process by taking advantage of nano composite three-layer structure. Results reveal that particles are uniformly dispersed in PI matrix. The nano silica can improve the conductivity and dielectric constant of PI film and influence the electron mobility. What’s more

the withstand voltage strength of Si O2/PI film ascends and then declines（5% reach the top）while at the same time corona resistant ascends with the increase of particle content of Si O2. Si O2 nano particle would generate in sample abundant organic-inorganic interface and sophisticated complex affects the electron’s action in dielectric

which at last changes the insulting property of PI film.