Despite　quick　development　of　perovskite　light-emitting　diodes　(PeLEDs)　during　the　past　few　years,　the　fundamental　mechanisms　on　how　ion　migration　affects　device　efficiency　and　stability　remain　unclear.　Here,　it　is　demonstrated　that　the　dynamic　redistribution　of　mobile　ions　in　the　emissive　layer　plays　a　key　role　in　the　performance　of　PeLEDs　and　can　explain　a　range　of　abnormal　behaviours　commonly　observed　during　the　device　measurement.　The　dynamic　redistribution　of　mobile　ions　changes　charge-carrier　injection　and　leads　to　increased　recombination　current;　at　the　same　time,　the　ion　redistribution　also　changes　charge　transport　and　results　in　decreased　shunt　resistance　current.　As　a　result,　the　PeLEDs　show　hysteresis　in　external　quantum　efficiencies　(EQEs)　and　radiance,　that　is,　higher　EQEs　and　radiance　during　the　reverse　voltage　scan　than　during　the　forward　scan.　In　addition,　the　changes　on　charge　injection　and　transport　induced　by　the　ion　redistribution　also　well　explain　the　rise　of　the　EQE/radiance　values　under　constant　driving　voltages.　The　argument　is　further　rationalized　by　adding　extra　formamidinium　iodide　(FAI)　into　optimized　PeLEDs　based　on　FAPbI(3),　resulting　in　more　significant　hysteresis　and　shorter　operational　stability　of　the　PeLEDs.