Poly(aryl ether ketone) (PAEK) is a kind of thermoplastic resin that can be used for toughening thermoset resins. In this work, we modified PAEK by introducing naphthalene group into the polymer chain. The physicochemical properties of the prepared thermoplastic resin were investigated by a few methods. The molecular weight was determined by gel permeation chromatography (GPC), and the glass transition temperature was measured by differential scanning calorimetry (DSC). The microstructure of the blends doping with bismaleimide resin was examined by scanning electron microscopy (SEM). The SEM micrographs show obvious phase separation structures, which are typical characteristics of reaction-induced phase separation. The present results may provide information for designing thermoplastic polymers used for toughening bismaleimide resins.