To limit the settlement of double-layered soft ground formed by the dredging blow filled soft soil and the natural deposited soft soil, precast square pile and deep mixing column with expanded head are combined to improve the ground. A field test was carried out in Lianyungang port railway project. The ground settlement, differential settlement between pile and soil, layered soil settlement, deep horizontal displacement, pore water pressure and loading ratios between pile, soil and column were studied. The test results show that the column-soil stress ratio of composite foundation increases with the external loadings. The foundation deformation occurs in the filling stage, and the settlement in the preloading period continues and tends to be stable. Filling load transfers to the precast square pile and T-shape deep mixing column by 56.2% and 20.4% of the total loadings, respectively, the replacement ratio and soil condition is the main influential factor for the proportion of loadings.