Perovskite　solar　cells　(PSCs)　have　gained　tremendous　research　interest　because　of　their　tolerance　of　defects,　low　cost,　and　facile　processing.　In　PSC　devices,　PbI2　has　been　utilized　to　passivate　defects　at　perovskite　film　surfaces　and　GBs;　however,　a　systematic　mechanism　of　PbI2　in　situ　passivation　for　enhancing　the　solar　cells　efficiency　has　not　been　fully　explored.　Here,　this　work,　we　systematically　studies　the　effect　of　the　precise　PbI2　ratio　and　the　PbI2　in　situ　passivation　mechanism　based　on　trap　density,　carrier　lifetime,　Fermi　level,　and　so　forth.　This　study　finds　the　appropriate　ratio　of　I/Pb　to　be　around　2.57:1　using　energy-dispersive　spectroscopy.　After　the　moderate　excess　PbI2　in　situ　passivation,　the　trap　density　is　reduced　from　6.12　X　10(16)　to　3.38　x　10(16)　cm(-3),　and　the　carrier　lifetime　is　extended　from　168.35　to　368.77　ps　by　using　fs-TA　spectroscopy.　This　result　indicates　that　the　moderate　excess　PbI2　in　situ　passivation　can　reduce　the　trap　density　and　suppress　the　nonradiative　recombination.　The　efficiency　of　solar　cell　has　shown　a　nearly　11.3%　improvement　of　19.55%　for　an　I/Pb　ratio　of　2.57:1　compared　with　2.69:1.　It　also　demonstrates　that　the　efficiency　of　PSCs　can　be　enhanced　effectively　by　PbI2　in　situ　passivation.