Approaches to promote the post-cure reaction of polyimide with pendant phthalonitrile unit (CN-PI) were first realized by introducing of hydroxyl groups via chemical copolymerization or physical blending. The post-cure reaction of the CN-PI was performed at 250 or 300 °C, and monitored by various techniques, such as infrared spectroscopy (IR), differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), scanning electron microscopy (SEM), and thermogravimetric analysis (TGA). A significant result was found in that the trend of the increasing glass transition temperature T g with nitrile conversion followed DiBenedetto’s equation. This provided a chance to deeply discuss the relationship between molecular structure, segmental mobility, and cure behavior of CNPI. Chemical structure changes and morphology evolution of the polymer during post-cure process were investigated by IR, WAXD, and SEM. The thermal and mechanical properties of polymer films showed it to be a thermally stable and strong material.