The　migration　and　volatilization　of　cations　under　environmental　stress　seriously　affect　the　stability　of　perovskite　solar　cells　(PSCs),　hindering　their　large-scale　deployment.　In　this　study,　a　novel　strategy　of　moisture-triggered　in　situ　polymerization　is　first　proposed　for　enhancing　PSCs　stability.　Ethyl　α-cyanoacrylate　monomers　are　introduced　into　the　perovskite　layer,　and　the　anionic　polymerization　is　rapidly　triggered　by　the　moisture,　forming　a　dense　poly(ethyl　α-cyanoacrylate)　(PECA)　layer　that　encapsulates　grain　boundaries.　This　effectively　suppresses　cation　volatilization　and　mitigates　defects　at　grain　boundaries.　As　a　result,　PECA-encapsulated　PSCs　achieve　a　PCE　of　24.12　%,　maintaining　exceptional　phase　stability　during　the　photothermal　aging　tests.　Unencapsulated　devices　retain　over　90　%　of　their　initial　efficiency　after　2000　h　in　the　air.　Thus,　such　a　multifunctional　internal　encapsulation　strategy　can　hold　great　potential　for　the　development　of　environmentally　stable　PSCs.　©　2025　Elsevier　B.V.