This research aimed to develop a combustion synthesis method to decrease curing temperature of benzoxazine with enhanced thermal stability and mechanical properties. In(NO3)3 was selected as an oxidant and acetylacetone was employed as a fuel to form a combustion system. The mechanism of combustion synthesis that involved curing benzoxazine was studied, and thermal stability and mechanical properties of cured products were discussed. It was revealed that the proposed method could significantly reduce the curing temperature of benzoxazine, and the initial curing temperature dropped to below 100 °C. Compared with polybenzoxazine, proposed method indicated an improved thermal stability, and the highest carbon yield at 800 °C was increased by 25.0%. In addition, the possible effects of the proposed method on coatings were investigated. The impact resistance, hardness, and surface adhesion were significantly improved to 7J, 6H, and 5B, respectively, with excellent anti-corrosive properties including a corrosion rate of 1.39 × 10?3 mm/year, indicating its great application potential in coating. The formation of a high content of arylamine Mannich structures and a small amount of In2O3 after curing could justify the improvement of thermal stability and mechanical properties of polybenzoxazine. The proposed method not only provided a facilitate way to cure benzoxazine, but also showed to be significant for modulating the chemical structures and properties of cured benzoxazine.