Abstract: The effectiveness of soft soil improved by dry jet mixing pile (DJMP) composite foundation is fully studied through the field tests on high-speed railways. Under the geological conditions, 5-cm post-construction settlement of the soft soil improved by the DJMP composite foundation can be controlled in a short period. The compressibility of the soft soil treated by the DJMP is smaller than that by plastic drain belt both for crust and silt soil. The modulus of compressibility of silty soil improves greatly. The stress and differential settlement of the composite foundation can be adjusted by gravel mat. The pile doesn’t penetrate into embankment with gravel mat under embankment. Pore water pressure and settlement data measured in field tests indicate that the rate of consolidation is faster than that of natural soft soil. The ratio of stress between piles and soil changes with sites, time and load. The ratio of stress between piles and soil is about 5 under the stable load.