A blend system of hexa‐(4‐carboxyl‐phenoxy)‐cyclotriphosphazene (HCPCP) and bisphenol A aniline benzoxazine (BA‐a) was prepared, and its curing reaction mechanism and heat resistance properties were studied. The curing reaction mechanism of the blend was explored by differential scanning calorimetry, Fourier transform infrared spectroscopy, and modeling software using model compounds Ar‐COOH and hexachlorocyclotriphosphazene (HCCP). The heat resistance properties of the cured blends were studied by thermogravimetric analysis and dynamic mechanical analysis. The polymerization of benzoxazine was catalyzed by HCPCP through phosphazene ring and acid groups, and phosphazene played a predominant role. Compared with the materials with a single functional group (Ar‐COOH and HCCP), HCPCP containing two functional groups (phosphazene ring and acid) exhibited weaker catalytic effects, mainly due to the high molecular weight of HCPCP obstructing movement and causing steric hindrance. In addition, HCPCP had a positive effect on the thermal stability of polybenzoxazine from 250 to 400?°C. When the HCPCP content reached 3%, the cured blend had the highest glass‐transition temperature (222.2?°C), which is higher by 20?°C than that of cured benzoxazine.