Crystalline structure of materials plays a decisive role in light-matter interaction. Yet, despite its unprecedented progress, further efficiency boost of lead-based organic-inorganic perovskite solar cells is hampered by their greater bandgap than the optimum value according to Shockley-Queisser limit.1-11 Here, we report the experimental achievement on the bandgap narrowing in formamidinium lead triiodide (HC(NH2)2PbI3) from 1.489 eV to 1.337 eV by modulating the lattice constants with modest hydraulic pressure up to 2.1 GPa, for the first time reaching the optimized bandgap for lead-based perovskites single-junction solar cells. Strikingly, such bandgap narrowing is accompanied with doubled carrier lifetime, and the enhancement in narrowed bandgap can be partially retained after the release of pressure. This work opens a new dimension in basic science understanding of structural photonics and paves an alternative pathway towards more efficient photovoltaic materials synthesis.