Abstract: A vibrof lotation method of strengthening sand shell of dams against earthquake is presented in this paper. Several vibroflotation tests were performed at Xingwang Dam. It has been known that the slope-effect and a large unsaturated zone in the dam are existed. Therefore, the vibroflotating sand shell of dams is different from the vibrof lotating soil foundation. The analysis of the observed data of the dynamic response of the dam under vibroflotation indicates that the vibratory acceleration and excess pore water pressure decrease rapidly as the radial distance from the vibroflot increase, and that the relationships between both of them and the radial distance from the vibroflot are of power function. The study on the mechanism of the densification by vibroflotation shows the three factors cause the compaction of the saturated loose sand via vibrof lotation: the first, the impulsive radial movements squeeze sand into the interior of slope; the second,the vibrof lotations induce excess pore water pressure and set the sand slope to suffer liquefaction; and the third, sand settles down and consolidates. And then a set of technical parameters of vibrof lotation are presented. To use these parameters and make the compacted zone of sand shell saturated with water is the key to increasing the compaction effect. This is the major premise for compacting the Xingwang Dam's sand shell to a relative density of about 0.8 and making the number of blows of S.P.T. more than forty.The main experiences of the application of vibroflotation at Xingwang Dam are also described briefly in the paper. In short, the vibroflotation method is not only safe and facile, but also economical and efficient. Therefore, it's a good artificial method for earthquake resistance of the existing earth dams with weak sand shells.