<p>Poly(aryl ether sulfone) with fatty-acid side chains that crosslink with epoxy resin improves the interfacial compatibility between poly(aryl ether sulfone) and epoxy resin. Hydroxyl-terminated phenolphthalein-based poly(aryl ether sulfone) (PPES-OH) was blended with fatty-acid side-chain-modified phenolphthalein-based poly(aryl ether sulfone) (PPES-TA), with the goal of further enhancing the toughening effect on epoxy resin. In this study, PPES-OH, PPES-TA, and a composite poly(aryl ether sulfone) (PESP-TA) were synthesized. Their molecular structures and thermal properties were characterized using proton nuclear magnetic resonance spectroscopy (¹H-NMR spectroscopy), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC). Subsequently, PPES-OH, PPES-TA, and PESP-TA were introduced into the anhydride-cured epoxy system to evaluate their toughening effects on epoxy resin. The curing behavior of the epoxy resin blends was investigated using DSC, which also enabled the exploration of the corresponding curing mechanisms. The thermal and mechanical properties of the toughened systems were characterized. Scanning electron microscopy (SEM) was used to observe the impact fracture surfaces of the resin, which revealed ‘fish-scale’ structures and shear bands in the resin system after curing. These findings demonstrate that similar thermoplastic chains become entangled with one another, forming additional physical cross-links. This enhanced the interfacial compatibility between the thermoplastic and thermoset resins, which, in turn, significantly improved the impact toughness and elongation at break of the system. In summary, PESP-TA has emerged as a reactive thermoplastic toughening agent that is feasible for preparation and has significant practical application potential.</p>