Perovskite decomposition arising from water permeation and heat induced crystal expansion is a major obstacle restricting the long-term durability of perovskite solar cells (PSCs). Herein, a polymerizable methyl acrylate (MCE) was employed as dopants in the deposition of perovskite thin films. Owing to the in situ formed polymer network, the environment moisture can be retained on the perovskite surface as the formation of a thin layer of perovskite monohydrate to prevent their deep penetration and transverse spread, and the heat tolerance of the perovskite was also improved because of the anchor structure between Pb2+ and -C═O groups and the agglomeration effect of the polymerized MCE. Moreover, MCE can coordinate with Pb2+ ions and some of them were volatilized during crystallization, resulting in preferred crystal orientations and suppressed nonradiative recombination. As a result, an excellent efficiency up to 21% with improved stability of MAPbI3 PSC was achieved.