A novel multiarm aluminum salt of pentaerythrityl ester of tetra(carboxyethylmethylphosphinic acid) (Alcpp) was synthesized and added into thermoplastic polyester elastomer (TPEE). Thermal analysis, evolved gas analysis (TGA-FTIR), flammability tests (LOI, UL94), microcombustion calorimeter (MCC) and chemical analyses of residues (SEM-EDX) were used. Alcpp provides TPEE with desired flame retardancy and anti-dripping property and shows a good compatibility with TPEE. TGA-FTIR and SEM-EDX analysis show that Alcpp can depress the heating release and promote the char forming and acts mainly in condensed phase. In addition, Alcpp combining with MPP achieves better fire-resistant performance than Alcpp alone. When total additives is kept 28 wt%, 25 wt% Alcpp combining 3 wt% MPP exhibits the optimum synergy by increasing char yield and depressing heat release and flammability. The reason for the synergy is due to a strong compact carbonaceous char resulting from reaction between Alcpp and MPP, which effectively prevent the dripping of the TPEE during combustion.