High mechanical strength, thermal stability, and flame retardancy are three crucial criteria for high-performance polymers to be suitable for aerospace applications. Most polymers, however, cannot meet the three criteria simultaneously. Herein, phosphonium bromide-terminated poly(ether imide)s (PEI-PhPPh3Br) simultaneously possessing high mechanical strength, thermal stability, and flame retardancy were synthesized by functionalizing dianhydri-determinated poly(ether imide)s (PEI-DA) with triphenyl-4-aminophenylphosphonium bromide. With the judiciously designed end group, PEI-PhPPh3Br exhibited excellent tensile properties, thermal stability, and flame retardancy. Importantly, PEI-PhPPh3Br with a molecular weight of 12 kDa [PEI-PhPPh3Br (12k)] showed a tensile strength of 109 +/- 4 MPa and a Young's modulus of 2.75 +/- 0.12 GPa, much higher than those of the noncharged PEI analogue. Additionally, PEI-PhPPh3Br (12k) showed outstanding flame retardancy, better than the state-of-the-art commercial PEIs, as evidenced by the high limiting oxygen index of 51% and high char yield of 60% at 980 degrees C. The study herein provides a highly effective strategy to simultaneously improve mechanical strength, thermal stability, and flame retardancy, which are three important properties rarely possessed by most polymers.