Metal　magnetic　memory　(MMM)　method,　a　non-destructive　inspection　method,　has　a　function　of　early　detection.　The　research　about　MMM　has　been　in　the　early　stage.　Its　originating　mechanism　hasn＇t　been　illuminated　clearly　and　there　is　not　a　detection　standard　which　can　be　widely　adapted　to　the　practical　engineering.　To　seek　the　mechanism　of　MMM　and　provide　experimental　data　for　establishing　the　detection　standard,　the　characteristics　of　change　of　perpendicular　magnetic　intensity　on　the　surface　of　Q235　steel　specimens　during　simple　tension　are　studied　in　this　paper.　Tensile　testes　to　specimens　sized　300　mm×25　mm　×　3　mm　in　the　universal　electronic　test　machine　have　been　carried　out.　And　the　process　of　plastic　propagation　is　especially　observed　and　recorded.　In　all　stages,　the　magnetic　field　distribution　is　obtained　by　magnetic　intensity　measurement　instrument.　Meanwhile,　the　location　of　slip　line　is　noted.　The　experimental　study　shows　that　there　is　not　connection　between　elastic　deformation　and　turn　of　magnetic　intensity　on　the　specimen＇s　surface　and　that　local　plastic　deformation　can　arouse　the　magnetic　turn.　The　locations　of　slip　lines　accord　with　the　magnetic　turn　points　that　can＇t　be　affected　by　the　earth　magnetic　field.