The separator is an important component for energy storage devices. At present, the membranes for supercapacitors are rare, especially the ones that work with an alkaline electrolyte. Herein, a type of flexible and highly porous polymer hybrid nanofiber membrane was prepared via a facile electrospinning process and served as a separator for supercapacitor’s work with strong alkaline electrolyte. As obtained, polypoly(2,5-benzophenone)bibenzopyrrolone/polyimide (PBPY/PI) membrane showed good thermal stability, high mechanical strength, large electrolyte uptake (452%), and fast ion conductivity (0.68 mS/cm). Moreover, PBPY/PI membrane exhibited excellent alkali resistance. The supercapacitor assembled with PBPY/PI membrane showed much better performance than those assembled with a commercial polypropylene membrane and electrospun polyimide nanofiber membrane and was found without capacitance loss after charge/discharge at 30 A/g for 10,000 cycles at 80 °C. The PBPY/PI membrane is a good candidate with temperature resistance and alkali resistance for high-performance supercapacitors.