On　the　basis　of　the　membrane　penetration　theory,　an　experimental　investigation　of　the　instantaneous　mass　transfer　coefficient　at　the　steam-liquid　interface　during　water　film　flash　evaporation　was　conducted.　The　range　of　the　height　and　the　superheat　of　water　film,　and　the　pressure　of　water　in　the　vacuum　chamber　was　20-100　mm,　10-20°C,　and　0.005-0.015　MPa,　respectively.　The　results　show　that　the　mass　transfer　coefficient　varies　from　0.08　m/s　to　16.77m/s　under　the　experimental　conditions.　The　coefficient　decreases　with　the　increase　of　the　initial　superheat　or　the　initial　height　of　water　film.　However,　during　the　flash　evaporation,　the　instantaneous　mass　transfer　coefficient　decreases　quickly　with　the　decrease　of　the　height　of　water　film　or　the　increase　of　the　pressure　in　the　vacuum　chamber.