Abstract: Polyimide and their composite dielectrics have attracted extensive attention of researchers because of their excellent comprehensive properties. With the first synthesis of commercial polyimide films by DuPont and the development of UHV transmission projects in China, polyimide and their composite dielectrics are widely used in the insulation field of high-voltage electrical equipment, and play an important role in the safe and stable operation of electrical equipment and even the entire power grid. However, the low breakdown strength can no longer satisfy the development requirements of thinness and high-power of high-voltage electrical equipment. In this paper, the relationship between the microscopic characteristics of the polymer and the breakdown strength is discussed from three aspects of carrier traps, free volume and blocking effect. Then, the research progress in insulation breakdown characteristics of polyimide composite dielectrics is introduced emphatically from two aspects of organic/inorganic composite and all-organic composite. Finally, combined with the future development needs of high-voltage electrical equipment, the key issues and future research directions of high-insulating polyimide composites are summarized and prospected.