In order to prepare aromatic nitrile-based resins with improved curing reactivity, thermal and thermomechanical properties, and to clarify the difference in curing activity between dicyanoimidazole and phthalonitrile groups, a novel class of autocatalytic aromatic nitrile-based resins named PNDCI which contains both dicyanoimidazole (DCI) and phthalonitrile (PN) groups were designed and synthesized. A method for evaluating the reactivity of nitrile groups using the absorption wavenumber of FTIR was proposed. PNDCI monomers have demonstrated higher activity for self-promoted curing reactions than benzimidazole-containing phthalonitrile monomers (BIPN). The introduction of DCI can change the curing pathway of PN and inhibit the formation of phthalocyanine ring which is an important cured product of PN resins. The data of TGA, DMA, etc. revealed that the cured products of PNDCIs showed outstanding thermal stability, long-term thermo-oxidative stability at high temperatures (425°C) and thermal mechanical properties, which is favorable for the preparation of high temperature resistant resins.