CsFAMA-based　perovskite　solar　cells　(PSCs)　have　become　promising　photovoltaic　materials　due　to　their　excellent　photovoltaic　properties.　However,　the　inherent　instability　of　perovskite　severely　hinders　its　commercialization,　since　the　typical　ionic　nature　of　perovskite　can　cause　the　ionic　migration　and　volatilization　of　organic　components.　Herein,　allantoin　molecules　with　multiple　alternating　carbonyl　(C　=　O)　and　amino　(-NH2　or　-NH-)　groups　were　introduced　as　a　multidentate　additive　to　synergistically　anchor　the　perovskite　components　and　passivate　the　defects,　resulting　in　non-radiative　recombination　and　ion　migration　being　not　susceptible　to　occur.　Due　to　the　favorable　attraction　ability　of　allantoin　to　the　perovskite　precursor　monomers　and　the　formation　of　hydrogen　bonds　between　FAI/MAI　and　allantoin,　the　crystal　quality　of　perovskite　was　significantly　improved　and　the　bulk/　interface　defects　were　effectively　eliminated.　Specifically,　the　interaction　between　allantoin　and　FA+/MA+　can　suppress　their　deprotonations　and　prevent　the　volatilization　of　organic　cations　from　the　initial　precursor　solutions,　drastically　reducing　the　wastage　of　precursor　materials.　Consequently,　the　modified　devices　demonstrated　a　prominent　elevation　of　performance　with　a　higher　efficiency　of　21.49　%,　which　was　better　than　that　of　control　(19.67　%).　Moreover,　the　modified　perovskite　films　also　demonstrated　prominent　stability　under　the　continuous　illumination　and　moisture　aging　owing　to　the　reduction　of　trap　defects.　The　allantoin　additives　also　greatly　enhanced　the　hydrophobicity　of　perovskite,　and　the　modified　device　maintained　90　%　of　its　initial　efficiency　for　1680　h　at　10-20　%　relative　humidity　without　encapsulation.