As　one　of　the　most　promising　photovoltaic　materials,　the　efficiency　of　inorganic-organic　hybrid　halide　perovskite　solar　cells　(PSCs)　has　reached　25.5%　in　2020.　However,　the　stability　and　hysteresis　remain　primary　challenges　before　it　can　become　a　commercial　photovoltaic　technology.　Therefore,　those　issues　have　drawn　significant　attention　for　photovoltaic　applications.　In　this　work,　a　study　of　the　PSCs　hysteresis　improvement　is　presented　based　on　a　combination　of　first-principles　simulations,　scanning　electron　microscopy　images,　and　time-dependent　photocurrent　measurements.　It　indicates　the　hysteresis　led　by　the　ion　migration　and　accumulation　is　mainly　localized　at　the　two　interfaces:　one　is　between　electron　transport　layer　and　active　layer,　and　the　other　is　between　active　layer　and　hole　transport　layer.　Considering　the　massive　defects　at　the　grain　boundaries　(GBs),　they　lower　the　potential　barriers　significantly.　The　defect　density　at　GBs　is　therefore　reduced　via　the　in　situ　passivation　of　PbI2　crystals.　The　hysteresis　index　is　decreased　from　22.43%　down　to　1.04%,　and　results　in　an　improvement　in　efficiency　from　17.12%　up　to　20.10%.　Following　the　understanding　of　defect-induced　hysteresis,　an　approach　to　improve　the　hysteresis　is　provided,　which　can　be　integrated　into　the　fabrication　process　and　widely　applied　to　enhance　the　performance　of　PSCs.