Large high-speed landslide may produce a very high increase in temperature,and convert the water in sliding zone into vapour,creating a cushion which sustains the slide mass. As a result,effective normal pressure reduces significantly under the sliding surface,and the sliding mass acquired the high velocity. It was proved by calculation on Touzhai large high-speed rockslide in Yun' nan province that the water in sliding zone can be vaporized,and the value of steam pressure is calculated.