Based on a large number of data determined from the practical projects treated by vacuum preloading and the result of finite element analysis on the basis of Biot's consolidation theory,the factors including coefficient of consolidation, depth of the plastic drain, distance between the plastic drain, degree of vacuum and the preloading area, are studied which determine the lateral deformation of the foundation treated by vacuum preloading.The hypothesis of single well in Barron's theory of axially symmetric consolidation is also proved to be reasonable.