To expand the use of unsaturated polyester (UP), its flame retardation, thermostability, and glass transition temperature (T-g) must be improved. To achieve this purpose, a novel organophosphorus polymeric flame retardant, poly(hydroxyphenyl imino methyl phenol spirocyclic pentaerythritol diphosphonate) (PPISP), containing both Schiff base and spirocyclic diphosphate structures, has been designed and synthesized. When PPISP is incorporated into the UP matrix, the cured UP thermoset containing 20 wt% PPISP exhibits a much higher initial decomposition temperature (T-5%) and T-g than pure UP. Simultaneously, the thermoset composite possesses a limiting oxygen index (LOI) value of 28.2% and achieves a UL-94 V-0 rating as well as sharply decreases the peak heat release rate by 60%. These results demonstrate that PPISP dramatically enhances the flame re-tardancy of UP. Additionally, the flame-retardant UP (FRUP) exhibits outstanding durable flame-retardance and water resistance. The catalytic charring effect and crosslinking behavior of PPISP are confirmed by thermo-gravimetric analysis (TGA) and TGA/DSC simultaneous thermal analysis. These analyses reveal that the Schiff base structure and spirocyclic diphosphate groups jointly contribute to the enhancement of the thermal stability of UP. The char layer of UP containing PPISP forms an extraordinary network structure with a stable graphitic microstructure, which acts as a protective barrier during pyrolysis and combustion. The TG-IR and Py-GC/MS results confirm that PPISP mainly takes effect in the condensed phase via catalytic carbonization and aromatization.