AbstractBandgap‐tunable mixed‐halide perovskite materials have attracted considerable interest because of their indispensability as top counterparts in tandem solar cells. However, the soft and disordered lattice always suffers from severe phase segregation under illumination, which is particularly susceptible to residual lattice strain. Herein, we report a strain regulation strategy by using alkenamides terminated Ti3C2Tx MXenes as an additive into perovskite precursor. Apart from the role of a template for grain growth to obtain high‐quality films, the stretchable alkyl chain promotes lattice shrinkage or expansion to form an elastic grain boundary to eliminate the spatially distributed stain and shut down ion migration channels. As a result, the all‐inorganic perovskite solar cells based on CsPbIBr2 and CsPbI2Br halides achieve prolonged device stability under harsh conditions and the best power conversion efficiencies up to 11.06% and 14.30%, respectively.