The polyimide/multi-walled carbon nanotubes (MWNTs) nanocomposite films were prepared by means of ultrasonic blending and in-situ polymerization. MWNTs were modified with mixed strong acid in order to improve the chemical compatibility between polyimide matrix and MWNTs. The effects of the incorporation of MWNTs on the electrical property were researched in this study. The results showed that with the increase of the content of acid-modified MWNTs, the dielectric strength decreased gradually and had a sharp drop when the addition content was close to 0.6 wt%, the dielectric constant and the dielectric loss factor increased obviously. With the rise of the test frequency, the dielectric constant declined slowly and the dielectric loss factor increased. The corona resistance time of the nanocomposite films was all higher than that of the pure polyimide.