Research conclusions ： It was concluded that ： ( 1 ) It was the direct reasons causing " soil arching effect" in the embankment fill of the big difference between the stiffness of piles and the stiffness of soils, and the soil arching effect mainly depended on the differential stiffness. (2) For this kind of composite foundation, the settlement model included the equal - settlement zone in the embankment，the soil arching effect zone in the embankment，the negative friction impact zone, the positive friction impact zone and the equal - settlement zone in the substratum under the bottom of piles. The stiffness of the cushion had a main impact on the settlement difference between piles and soils. (3) For this kind of composite foundation, the average earth pressure on the top of CFG piles was 196 times bigger than the average one on the soil - area between piles，and that on the top of MIP piles was 22 times bigger than that on the soil - area between piles. The ground bearing capacity was obviously improved by MIP piles. (4) By this kind of composite foundation combined with preloading method for improving deep soft soil subgrade，96％of the total settlement was eliminated，and the settlement of post-construction was around 2．0 mill．Therefore， it met the requirements of stability during the operation of high-speed railway．(5)The research results can provide a good guidance for the improvement of deep soft soil roadbed in high-speed railway and highway．