In the course of treating soft foundation by "vacuum preloading", the consolidation of soft foundation is not brought about by the "pre-pressing" of "vacuum load", but by the formation of vacuum seepage field and the "infiltrating-concretion" caused by vacuum seepage field. In the paper, the forming mechanism of vacuum seepage field is discussed in detail, including the physical models of pore water moving and soil consolidating. In the discussion of the soil consolidation model, the "effective stress" is expanded into two parts, which are the vertical effective stress and the horizontal effective stress. Based on this, a bi-spring consolidation model has been put forward. By analyzing the forming process of the vacuum seepage field from the aspect of the "effective stresses" developing, the bi-spring consolidation model is found to fit the essential of the effect of "vacuum preloading" on soft foundation.