Abstract: In order to improve the problems of low porosity, low electrolyte uptake of commercial separators, poor heat resistance and thermal dimensional stability, Polybenzimidazole (PBI) was used to modify polyimide (PI). The PBI∶PI=0.3∶1.0 (mass ratio) composite fiber separator was prepared by the method of high-voltage electrostatic spinning. The microscopic morphology, porosity, electrolyte uptake, thermal performance, electrochemical performance and battery performance of the composite fiber separator were studied. The performance of composite fiber separator with mass ratio of PBI∶PI=0.3∶1.0, PI fiber separator and polypropylene (Celgard 2400, PP) separator were compared. The results show that the composite fiber separator with PBI∶PI=0.3∶1.0 has a porosity of 82% and electrolyte uptake of 618%; in an air atmosphere, there is no size shrinkage at 300℃; in a nitrogen atmosphere, the decomposition temperature is above 400℃, and the residual mass at 800℃ is more than 50%. The ionic conductivity reaches 1.29×10⁻³ S/cm, which is almost an order of magnitude higher than PP separatos; the interface impedance is 489.34 Ω, which is 17% lower than PP separator; the electrochemical stability window is increased to 5.05 V, which is 19% of the PP separator; CR 2032 battery assembled with PBI∶PI=0.3∶1.0 composite fiber separator shows excellent battery performance. After high current discharged, cells properties remain stable, initial discharge capacity is 130.01 mA·h/g, capacity retention rate is 98.91% after 100 cycles of 1 A/s, which is better than PP separator cells.