Abstract: Polyimide (PI) nanofiber membranes remain poorly understood in terms of their thermal conductivity behavior under complex and variable operating conditions. In this study, a PI nanofiber membrane fabricated by electrospinning, with an average fiber diameter of approximately 500 nm, was systematically investigated. Thermal conductivity was measured using the guarded hot plate method under mechanical deformation, wide temperature ranges, and varying ambient pressures, and the variation patterns were analyzed. The results indicate that the PI nanofiber membrane exhibits low thermal conductivity and good compression resilience, allowing it to maintain relatively stable thermal protection performance even under structural deformation. The thermal conductivity of the material is sensitive to ambient pressure: as pressure increases, the contribution of gaseous heat transfer leads to a significant increase in thermal conductivity, whereas under high vacuum conditions, radiative heat transfer becomes the dominant mechanism. Under equivalent areal density, the PI nanofiber membrane and multilayer insulation (MLI) structures exhibit different thermal insulation performances depending on the ambient pressure. MLI structures demonstrate superior insulation performance under high vacuum, whereas the PI nanofiber membrane performs better within a specific intermediate pressure range. However, at atmospheric pressure, the thermal conductivity of the PI nanofiber membrane is higher than that of MLI. This study provides a reference for their application in spacecraft thermal control systems.