Abstract: This paper presents a flexible electrical insulating composite film-DMD. Through an analysis of the film structure, a series equivalent circuit model is designed. By using this model, dielectric constant and strength are solved in not oil impregnated and oil impregnated DMD film and formulae are given. The formulae show that the dielectric constants increase obviously after impregnation and the dielectric strength of impregnated DMD film is higher than that of the not oil impregnated DMD film. The electric field strength on polyester material increases and that on impregnated material decreases with the increase of the oil dielectric constant. Due to the high dielectric strength of polyester material, the dielectric strength of the transmission line is increased.