Cesium-based inorganic perovskites have emerged as promising light-harvesting materials for perovskite solar cells (PSCs) due to their promising thermal- and photo-stability. However, obstacles to commercialization remain regarding their phase instability. In this work, we report a facile and effective strategy to regulate the surface compressive strain via in-situ surface reaction to stabilize CsPbI3 perovskite. The use of a chelating ligand with a molecular configuration closely matching the integer multiples of the unit cell lattice parameters of CsPbI3 induces compressive strain at the surface of CsPbI3. The chemical bonding and strain modulation synergistically not only passivate film defects, but also inhibit perovskite phase degradation, thus significantly improving the intrinsic stability of inorganic perovskite. Consequently, enhanced power conversion efficiency (PCE) of 21.0% and 18.6% were respectively achieved in 0.16-cm2 lab-scale devices and 25.3-cm2 solar modules. Further, surface reaction enables PSCs with enhanced thermal and operational stability; these devices retain over 95% of their initial PCE after damp-heat tests (i.e., in 85 ℃ and 85% R.H. air) for 2000 h, and remain 99% of their initial PCE after operating for 2000 h, representing one of the most stable inorganic PSCs reported so far.