Lithium−sulfur batteries (LSBs) have broad application prospects in high density energy storage. Nevertheless, LSBs present serious safety concerns and rapid capacity decay due to the flammability and contractility of commercial separators at high temperatures and lithium polysulfide (LiPSs) dissolution. In view of this, a multifunctional modified separator is developed by combining a commercial polypropylene (PP) separator with a polyimide (PI)-Super P functional coating and a rigid nonflammable PI matrix. This PI/PP/PI-Super P separator provides multiple advantages: (i) strong physical blocking and chemical trapping/conversion ability for LiPSs, (ii) high flame-retardancy and dimensional stability, and (iii) superior electrolyte wettability and high efficiency lithium dendritic growth inhibition. As a result, the LSBs assembled with PI/PP/PI-Super P separators and pure sulfur cathodes exhibit an ultrahigh discharge specific capacity of 1611.8 mAh/g and excellent cycling performance (the average capacity attenuation within 100 cycles at 1 C is 0.469%) at 80 °C, which exceed the advanced results reported previously. More importantly, even when sulfur and organic electrolyte are adsorbed on PI/ PP/PI-Super P, the modified separator still shows good flame retardancy. Additionally, this modified separator is also suitable for LSBs with high sulfur loading. This work offers an innovative design concept for safe and high-performance LSBs.