Several viewpoints of vacuum preloading mechanism were analyzed. Action process of vacuum preloading was compartmentalized three phases. They were seepage consolidation in shallow soil and “surcharge” effect in deep soil by vacuum suction, consolidation stabilization in the shallow soil and transformation of “surcharge” to “negative pressure” in deep soil, seepage consolidation in deep soil under “negative pressure” and to balance. Action mechanism root of vacuum preloading was not negative pressure difference solely, which was produced by seepage field. The root was “positive pressure” and “negative pressure” together during different stage. The root also included the two pressures superposition under the especial phenomenon of vacuum preloading: water level falling. Under the mechanism, stress of soil was analyzed. Effective stress path began from line resembling that under surcharge in q-p plane. It would never reach line and after a short period it would develop paralleling p-axis. Horizontal contractive displacement of soil took place because “negative pressure” intensity was much stronger than “positive pressure” intensity. If surcharge effect equal vacuum preloading effect, soil under vacuum preloading reached yield more difficultly. Last, measured value was analyzed and the analysis of mechanism, stress was validated.