Postsynthetic modification (PSM) has been widely employed to enhance the functionality of metal–organic frameworks (MOFs) for applications such as gas storage, light harvesting, and catalysis. However, the critical role of framework flexibility in enabling diverse modification pathways and influencing framework properties remains underexplored. In this work, we demonstrate that flexible MOFs offer unique advantages for PSM, using the flexible PCN-128 as a platform for both chemical and mechanical modifications to enhance photocatalytic CO2 reduction activity. Mechanical force induces piezochromic behavior, while sequential postsynthetic annulation reactions extend the π-conjugation of aromatic ligands within PCN-128, both modulating the energy band positions and enhancing visible light absorption. Single-crystal-to-single-crystal transformations were realized throughout the PSM process, offering valuable insights into the transformation process and structure-property relationships. Furthermore, metallo-phthalocyanine-based ligands (M-TcPC, M = Fe, Co, Ni) were introduced as catalytic sites for CO2 reduction via PSM strategies. By optimizing the ratio of photosensitizers to catalytic sites, the modified catalysts achieved a remarkable CO2 reduction rate of 838 μmol·g–1·h–1 under visible light irradiation. This study highlights the untapped potential of flexible MOFs in applying postsynthetic modification strategies to develop advanced photocatalysts for sustainable energy conversion.