The detrimental shuttle effect of lithium polysulfides (LiPSs) and the combustible features of commercial separators have hindered the practical application of lithium-sulfur (Li-S) batteries. Herein, a robust flame retardant fluorinated polyimide (F/PI) nanofiber separator has been designed for high-temperature Li-S batteries. The introduction of electron-withdrawing trifluoromethyl (-CF3) groups into the PI nanofiber not only increases its porosity to ameliorate the electrolyte uptake, electrolyte retention, and electrolyte wettability but also boosts its polarity to offer better affinity with LiPSs. More importantly, the existence of abundant -CF3 groups on the PI nanofiber can afford an electronegative environment to accelerate the transmembrane transfer of Li+ and simultaneously repel the negatively charged polysulfide ions. As a result of these advanced structural characteristics, a high initial capacity of 1512 mA h g-1 at 0.1C, an outstanding rate performance of 532 mA h g-1 at 5C, and a superior cycling stability with a capacity retention of 95.6% after 500 cycles at 1C can be achieved. Additionally, the flame-retardant F/PI nanofiber separator also delivers excellent cycling and safety performance at high temperature, holding a promising prospect for future commercialization.