In this paper, a new kind of composite polymer electrolyte has been successfully prepared by casting lithium bis(oxalate)borate–succinonitrile–polyethylene oxide (LiBOB–SN–PEO) polymer electrolyte solution into porous polyimide (PI) nanofibrous films. The obtained LiBOB–SN–PEO–PI composite polymer electrolyte has been characterized by X-ray diffraction, scanning electron microscopy, differential scanning calorimetry and electrochemical impedance spectroscopy technologies in detail. It is found that the interlaced PI fibres in the framework play an important role in achieving a superior high-temperature-tolerance characteristic up to 170 °C for the LiBOB–SN–PEO–PI composite polymer electrolyte, which is much higher than that (ca. 100 °C) of traditional PEO-based electrolytes. The composite polymer electrolyte also exhibits outstanding high-temperature stability, ensuring a long-term service life in high-temperature lithium-ion batteries.