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Perylene polyimide networks are currently one of the most promising metal-free photocatalysts due to their excellent electrical and optical tunability. However, the disordered aggregation state structure in perylene polyimide resulting from the traditional imidization process is detrimental to the separation and migration of the photogenerated charges, which in turn greatly hampers its photocatalytic activity. Herein, we report a facile approach to construct highly crystalline (∼98%) perylene polyimide photocatalysts via cascade reactions involving the assembly of perylenetetracarboxylic acid-melamine monomer salt crystals (PTA-MA), followed by solid-state imidization of the PTA-MA monomer salt. Meanwhile, oxygen vacancies (OVs) were introduced during the polycondensation of the PTA-MA monomer salts. By adjusting the polycondensation temperature, perylene polyimide photocatalysts (PTA-MA-Tn) with varying crystallinity and contents of OVs were obtained. Benefiting from its highly ordered structure and optimized OVs content, the as-prepared perylene polyimide photocatalyst (PTA-MA-200) obtains a suitable crystal structure and excellent photon-to-electron conversion efficiency and exhibits exceptional photoactivity in the photocatalytic valorization of benzylamines into imines. An approximately 10-fold increase in the rate of benzylamine photocatalytic conversion was observed in comparison to PTA. This study paves the way for the development of highly crystalline polyimide photocatalysts and has the potential to enhance the understanding of the structure–activity correlation in the context of applying perylene polyimide photocatalysts in practical scenarios.