When　the　power　conversion　efficiency　(PCE)　of　perovskite　solar　cells　(PSCs)　rapidly　approaches　that　of　commercial　solar　cells,　the　stability　becomes　the　most　important　obstacle　for　the　commercialization　of　PSCs.　Aside　from　the　widely　studied　slow　PCE　degradation,　the　PSCs　also　show　a　unique　rapid　PCE　degradation.　Although　the　degradation　due　to　oxygen　and　humidity　can　be　avoided　by　encapsulation,　that　due　to　bias　voltage,　light　and　heat　could　not　be　effective　suppressed　and　will　lead　to　considerable　degradation.　Usually,　the　rapid　PCE　degradation　is　believed　to　be　from　ion　migration.　However,　a　systematic　investigation　is　yet　to　be　carried　out.　This　work　quantitatively　and　systematically　investigated　the　relationships　between　external　fields　(bias　voltage,　light　or　heat),　ion　migration　and　device　performance.　By　comparing　the　performance　of　reference　PSCs　after　90　min　degradation　under　these　fields,　we　conclude　that　(1)　the　electric　field　affects　the　spatial　distribution　of　mobile　ions;　(2)　the　light　field　changes　the　mobile　ion　densities　and　drives　the　ion　migration;　(3)　the　heat　field　results　in　perovskite　decomposition　as　well　as　changing　the　mobile　ion　densities.　In　addition　to　the　analysis　of　the　reference　device,　we　experimentally　proved　that　the　improved　device　stability　upon　introducing　phenethylammonium　iodide　(PEAI)　or　poly-methyl　methacrylate　(PMMA)　layers　originates　from　the　inhibition　of　mobile　ion　density　and　migration.