Based on the changes of total stress and pore water pressure in the spring model, the mechanism of vacuum preloading consolidation is studied by theoretically discussing the consolidation behavior of soil in vertical and horizontal directions with different boundary conditions. The variation of effective stress and development of deformation are analyzed for the soil under vacuum preloading under stress paths of K0 consolidation and isotropic consolidation based on the boundary conditions, respectively. Meanwhile, the theoretical earth pressure coefficient (K0MC) at rest for soft soil is deduced based on modified Cam-clay model. With engineering applications, the correctness and effectiveness of the mechanism is confirmed by the application in large area of ground improvement for soft soil and statistical analysis of some engineering cases. It is found that the drainage boundary condition and the scale of treated area have effects on deformation of soil mass. For the middle part of the treated zone under large-area vacuum preloading, the variation of effective stress is not isotropic, but following the pattern of one-dimensional compression. According to the statistic results from some site investigations, one-dimensional compression equation (Cc-Cr method) can be applied to predict the settlement at the center of the treated zone when the one dimensional condition is found in the center of treated zone as the ratio between treated breadth and length of vertical drains larger than 4-5.