A novel intumescent flame retardant poly(phenyl phosphamide spirocyclic diethanolamine borate) (PPSDB) containing phosphorus, nitrogen and boron elements was synthesized successfully with diethanolamine borate and phenylphosphonic dichloride. The structure of PPSDB was characterized by infrared spectroscopy with Fourier transformation and nuclear magnetic resonance. As a new intumescent flame retardant, flame-retardant polyethylene (PE) composites were prepared by filling PPSDB with different fractions. The thermal stability of PPSDB and PE/PPSDB composites was investigated by thermogravimetric analysis. The results showed that the thermal stability of PPSDB was fit for processing PE composites, the initial decomposition temperatures of PE/PPSDB composites would decrease and the char residue would increase with an increase in the PPSDB fraction. The PE/PPSDB composite with 30 wt% PPSDB had the best combination properties; the limited oxygen index value was 29.2, and vertical burning reached UL-94 V-0 rating, and the tensile strength and notched impact strength were 18.6 MPa and 12.5 kJ m(-2) respectively. The morphologies of char residue showed a lot of protruding structures consisting of smaller protuberances, which prevented the combustion of composite and enhanced anti-dripping property effectively.