A main-chain-type polybenzoxazine with amic acid and benzoxazine groups as repeating units, generally termed as poly(benzoxazine amic acid) (poly(BZaa)), has been synthesized. Different from traditional main-chain polybenzoxazines, poly(BZaa) can undergo polymerization and imidization reactions to form cross-linked polyimide (cPI) after certain thermal treatment. cPI can further undergo a typical cyclization to give cross-linked polybenzoxazole (cPBO) after an additional thermal treatment. A model reaction is designed based on the reaction of 3-phenyl-2,4-dihydro-2H-benzo[e][1,3]oxazin-9-amine with a phthalic anhydride. Fourier transform infrared (FTIR) and proton nuclear magnetic resonance (H-1 NMR) spectroscopies are used to confirm the structure of model compound. The ring-opening polymerization behavior of benzoxazine monomers and main-chain polybenzoxazine is studied by differential scanning calorimetry (DSC). Thermal properties of the cross-linked polymers are also studied by dynamic mechanical analysis (DMA) and thermogravimetric analysis (TGA).