The　frost　formation　under　magnetic　field　was　studied　in　this　paper.　Through　close　observation　the　water　droplet　condensation,　the　droplet　freezing　process　and　the　frost　shape　were　all　very　different　from　that　without　magnetic　field.　The　water　droplet　is　smaller　and　more　homogeneous.　The　frost　structure　is　weaker,　looser,　thin　and　easy　to　be　removed　under　the　magnetic　field.　A　theoretical　analysis　was　presented　to　explain　the　phenomenon.