Multicomponent copolymerization is an efficient approach for balancing the processability and thermal and mechanical properties of polymers. However, the copolymerization system has rarely been observed in single component resins. Here we report a terpolymerization system in a novel ortho-norbornene functional benzoxazine containing acetylene group (oHPNI-ac), where the chemical structures of oHPNI-ac are confirmed by H-1 and C-13 nuclear magnetic resonance (NMR) spectroscopy and Fourier transform infrared (FT-IR) spectroscopy. The polymerization behaviors are investigated by differential scanning calorimetry (DSC), in situ FT-IR, and solid-state C-13 NMR. In addition to the polymerization of the oxazine ring and acetylene group, oHPNI-ac exhibits an unexpected lower thermally activated polymerization temperature of norbornene functionality compared with other reported norbornene containing monomers or oligomers. Furthermore, rheological measurements are applied to reveal the evolution of the rheological behavior of oHPNI-ac during the polymerization at different temperatures. The resulting thermoset derived from oHPNI-ac possesses the combined advantages of low-temperature polymerization, easy processability, high thermal stability, and low dielectric constant, indicating its potential applications in aerospace, electronics industries, and other areas requiring a high performance polymeric matrix.