Vanadium　dioxide　(VO2),　due　to　its　electrically　induced　metal-to-insulator　transition　with　dramatic　changes　in　electrical　and　optical　properties,　is　considered　to　be　a　powerful　material　for　electro-optical　devices.　However,　there　are　still　some　controversies　about　phase　transition　mechanism　under　voltage.　Here,　based　on　optical　characterizations　on　VO2　crystal　nanofilm　during　the　whole　process　of　phase　transition,　temporal　evolution　and　spatial　distribution　of　changes　in　electricity,　optic　and　temperature　are　investigated　simultaneously,　to　explore　the　mechanism.　The　variations　of　Raman　spectrum　and　reflected　spectrum,　and　changes　in　current　and　temperature　are　evidences　for　occurrence　of　phase　transition,　which　exhibit　different　changing　behaviors　with　time　and　space.　These　results　offer　a　better　understanding　of　the　phase　transition　mechanism,　implying　that　lattice　structure　of　VO2　changes　gradually　after　applying　voltage　until　the　structure　is　completely　converted　to　metallic　structure,　which　causes　a　rapid　increase　in　carrier　density,　resulting　in　a　rapid　change　in　current,　reflected　spectrum　and　temperature.　Temperature　rise　before　phase　transition　and　applied　electric　field　alone　are　not　enough　for　triggering　metal-insulator　transition,　but　these　two　factors　can　act　synergistically　on　structural　transformation　to　induce　phase　transition.