A polynitrile‐based phthalonitrile monomer with a low melting point containing an alkyl center, namely, 4,4′,4′′‐ [(methanetriyltris (benzene‐4,1‐diyl)) tris (oxy)] triphthalonitrile (MDTP), is synthesized from a nucleophilic substitution reaction of 4,4′,4′′‐trihydroxytriphenylmethane and 4‐nitrophthalonitrile. The MDTP monomer is cured at different temperatures 4‐(aminophenoxy)phthalonitrile (APPH) as the curing agent to give the MDTP polymers. The MDTP monomer and polymers are characterized by Fourier Transform Infrared spectroscopy (FTIR) and Nuclear Magnetic Resonance spectroscopy (NMR). Differential Scanning Calorimetric (DSC) analysis shows that the melting point of the monomer is 103 °C and the processing window is about 128 °C. The crosslinking degree of the resin varies with curing time and temperature, which affects the properties of resin. Its glass transition temperature (Tg) is higher than 400 °C. The resin possesses excellent thermal stability and mechanical properties. It loses 5 % weight at 511.2 °C and has a storage modulus of 1611 MPa at 400 °C.