Abstract: The research on mechanism of the soil improvement with field prototype monitoring is a normal practice.Based on the field monitoring in an experimental section of soft soil foundation improvement in an expressway construction project,the mechanism of treatment of cement mixing pile composite combined with Prefabricated Vertical Drains(PVDs) and preloading was analyzed.It was revealed that the downward thrust of short pile in the combined improvement method made the stress distribution on the pile top and soil more even,and the settlement of the pile top and soil more coordinative.As the existence of PVDs,the additional stress transminssion through the short pile would speed the consolidation in the lower soft soil layers.