The in situ reaction of a hydroxy group with a phthalonitrile system was carried out by the simple nucleophilic displacement of a nitro-substituent from 4-nitrophthalonitrile in a dipolar aprotic solvent, in a one-pot reaction. The hydroxy-containing phthalonitrile system (HPBD) was prepared by mixing 4-hydroxyphenoxy phthalonitrile (HPPH) and 1,3-bis(3,4-dicyanophenoxy)benzene (BDB), followed by heating. The curing behavior was studied using differential scanning calorimetry and dynamic rheological analysis, and the results indicated that the HPBD exhibits a large processing window (∼75 °C) and low complex viscosity (0.1–1 Pa s) at moderate temperatures. Fourier transform infrared spectroscopy (FT-IR) showed that polytriazine, polyindoline and phthalocyanine structures were formed during polymerization, and that the introduction of HPPH facilitated the curing reaction. Additionally, the prepared HPBD polymers showed outstanding thermal stability, a high modulus and a high glass transition temperature (T_g). After curing at 300 °C, the T_g of HPBD resin was raised to 410 °C. Postcuring effects on the thermal and dynamic mechanical properties were evaluated using thermogravimetric analysis (TGA) and dynamic mechanical analysis (DMA).