Research conclusions: The maximum reinforcement depth of compaction piles should not exceed 15m． The compact pile with the depth of 15m was used for the test subgrade． The surplus settlement volume of the subgrade that had been loaded by dead load for six months could meet the control requirement of the post － construction settlement for the ballastless track，and only a little settlement occurred to the reinforced layer of the foundation after submerging． And the collapsibility of the loess in the reinforced layer was eliminated fully． In the area with the thickness of the collapsible loess less than 15m，the compact pile is an effective way to reinforce the foundation for the settlement control．