Abstract: Addressing the issue of insufficient intrinsic toughness in poly(phthalazinone ether sulfone ketone) (PPESK) due to its strong molecular chain rigidity, this paper conducts research on the molecular design, synthesis, and properties of a high-toughness soluble poly(phthalazinone ether) resin. Based on molecular dynamics simulations, a theoretical model of the intrinsic flexibility of amorphous polymer molecular chains is constructed. The segment flexibility is evaluated using parameters such as free volume fraction (FFV), mean square displacement (MSD), cohesive energy density (CED), and intermolecular interaction energy. Adopting the design principle of "combining rigidity and flexibility", 4,4-bis(4-hydroxyphenyl)pentanoic acid (DPA) is used to partially replace dihydrazinopyranone biphenyl (DHPZ). On the basis of retaining the rigid skeleton of phthalazinone, flexible alkane side chains and carboxyl structures are introduced. A novel high-toughness soluble poly(phthalazinone ether) resin, PPEDSK, is prepared through a high-temperature solution nucleophilic substitution polycondensation method. Fourier Transform Infrared Spectroscopy (FTIR) and Nuclear Magnetic Resonance analysis (¹H-NMR) characterization results confirm the successful synthesis of the target polymer. Compared to PPESK, PPEDSK exhibits an increase in tensile strength by 16.7%, an increase in elongation at break by 20%, and an increase in notch impact strength by approximately 145.5%. Its glass transition temperature is 265℃, and its thermal decomposition temperature is 461℃, maintaining good heat resistance. It also demonstrates excellent solubility in various polar solvents. This research provides a material basis for the application of high-performance soluble poly(aryl ether) resins in the field of repeatedly large deformation folding/unfolding structures and advanced composite materials.