Abstract: The Semi-analytic method for vacuum preloading of a foundation with sand drains is improved in this paper, so that it can be also applicable to the case of surcharge preloading and multiple layered foundation. The Computation is simple and the area of improvement and the number of sand drains are not confined. This method takes into account the effect of clay viscoelasticity, 3-D consolidation, sand drain group, well resistance and smear. The calculated results of lateral movement of the improved method are checked more fairly well with field measurements than those of the unimproved ones.