Abstract: The sedimentary soft soil in Yangtze River floodplain is the fluvial facies sediment since the quaternary period, and mainly composed of silty clay, silty fine sand and fine sand, and is characterized by high compressibility, low strength and easy liquefaction. Therefore, the soft soil foundation needs to be improved during the construction of expressway, high-speed railway and other infrastructure. Thixotropic phenomenon, similar to liquefaction, is caused by improvement construction of soft soil foundation in the Yangtze River floodplain, and leads to the disaster of the soft soil foundation. The foundation treatment construction causes the soft soil to produce a damage phenomenon similar to liquefaction, resulting in catastrophic disaster to the soft soil foundation. The disaster phenomenon of the soft soil foundation is summarized and caused by the construction of deep cement mixing(DCM)column of high-speed railway in the South Yangtze River. The disaster phenomenon of the soft soil foundation mainly includes the sinkhole at the top of deep cement mixing column, the water and slurry spraying during the coring of deep cement mixing column, the cracks in soft soil foundation, the large amount and fast speed of settlement. In-site monitoring of earth pressure, pore water pressure and soil horizontal displacement of soft soil are carried in the construction of deep cement mixing column in the soft soil foundation. The disturbance of the soft soil is evaluated and the disaster criterion of the soft soil foundation is put forward during the construction of deep cement mixing columns. The property variation of soft soils Yangtze River floodplain is also analyzed.