Flexible perovskite solar cells (pero-SCs) are the best candidates to complement silicon solar cells in the photovoltaic market. However, their power conversion efficiencies (PCEs) and mechanical stability are far behind the industry standards because of the uncontrollable growth of perovskites on plastic substrates and intrinsically high Young’s modulus. We explored an in situ crosslinking bis((3-methyloxetan-3-yl) methyl) thiophene-2,5-dicarboxylate along with perovskite growth, and its coordination ability and crosslinking temperature enabled the fine regulation of the quality of perovskite in real time. The resultant perovskite film exhibits an enlarged grain size, compact stacking, and a preferential crystal orientation. Moreover, the crosslinked elastomer polymer gathered at the perovskite grain boundaries can effectively release the mechanical stress. As a result, the flexible pero-SC based on this perovskite film achieved a record PCE of 23.4% (certified 22.9%), which is comparable with that of the rigid device. Importantly, the flexible pero-SCs also display a robust bending durability.