In this paper, the prepolymerization of acetylene-functional benzoxazine was carried out by using nickel acetylacetonate hydrate/triphenyl phosphine as a catalyst at a low temperature. By this method, the reaction heat of acetylene polymerization was partially released and no ring-opening polymerization of benzoxazine occurred. After that, the prepolymer was cured stepwise to obtain the cured product according to a traditional curing process. As a comparison, another cured product was directly obtained from acetylene-functional benzoxazine by the traditional method without prepolymerization. The curing behaviors and the thermal properties of these two polybenzoxazines were investigated and compared by FTIR, DSC, DMA, and TGA. Their thermal decomposition behaviors were further studied by TGA-FTIR, Py-GC/MS, and SEM. The results show that the catalytic prepolymeriztion makes the whole curing process milder and more controllable in a broader processing window. Furthermore, more crosslinked-polyacetylene structures and benzene from the catalytic prepolymerization improve the thermal stability of the polybenzoxazines. Additionally, less amine compounds were detected in the prepolymerization system during the thermal decomposition process and the appearance of the residue was stable and controllable after pyrolysis.