A series of novel soluble phthalazinone-based copoly(aryl ether nitrile)s (PPBENs) were successfully synthesized by the nucleophilic substitution reaction of 2,6-difluorobenzonitrile (DFBN) with varying mole proportions of 4-(4-hydroxylphenyl)-2,3-phthalazin-1(2H)-one (DHPZ) and 4,4'-dihydroxybiphenyl (BP). These obtained copolymers had inherent viscosities between 0.88 and 1.26 dL/g in N-methyl-2-pyrrolidone (NMP) at a concentration of 0.5 g dL(-1) at 25 degrees C. Most of the obtained copolymers were soluble in common polar aprotic solvents, involving NMP, DMF, DMAc and chloroform, etc., at room temperature. The glass transition temperature was found to increase with increasing the DHPZ unit concentration in the copolymer main chain, ranging from 222 to 286 degrees C. Thermal gravimetric analysis showed that all the copolymers had 5 % weight loss temperature up to 521 degrees C and the high char yield above 65 % at 800 degrees C in nitrogen atmosphere. PPBEN2 exhibited the best mechanical properties which were measured using film samples. Its tensile strength and elongation at break were up to 137 MPa and 15 %, respectively. The thermal degradation process of PPBENs was discussed using thermogravimetric analysis (TGA) and FTIR. The apparent activation energy values were 273.5 and 274.1 kJ/mol which were calculated by Kissinger-Akahira-Sunose method and Flynn-Wall-Ozawa method, respectively. The results showed that the obtained copolymers possess excellent thermooxidation.