The　efficiencies　of　organic-inorganic　lead　halide　perovskite　solar　cells　(PSCs)　have　significantly　increased　so　far,　but　the　long-term　stability　of　PSCs　needs　to　be　urgently　improved.　In　this　work,　a　novel　and　convenient　strategy　is　developed　to　improve　PSCs＇　stability　by　introducing　thiourea　into　perovskite.　Thiourea　passivates　perovskite　by　the　formation　of　the　PbS　bond　on　the　outermost　layer　of　perovskite　and　lath-shaped　grain　among　perovskites,　which　contributes　to　the　improvement　of　oxygen,　light,　thermal　stability,　and　little　hysteresis.　The　mechanism　is　elucidated　by　the　density　functional　theory　calculation.　In　addition,　the　additive　of　thiourea　into　perovskite　significantly　improves　the　quality　of　perovskite　crystal　for　the　adduct　of　PbI2　with　thiourea　as　the　Lewis　base.　Devices　of　a　perovskite　film　with　the　thiourea　additive　achieve　high　efficiencies　of　19.57%　and　17.67%　for　active　areas　of　0.1　and　1.0cm(2),　respectively.　Most　importantly,　unencapsulated　devices　retain　98%　and　93%　of　original　efficiencies　after　two　months　under　air　conditions　for　active　areas　of　0.1　and　1.0cm(2),　respectively,　showing　excellent　stability.　The　present　work　provides　a　simple　and　effective　method　to　improve　long-term　stability　of　PSCs.