With the rapid development of high-rise buildings and infrastructure construction, higher requirements for foundations have been put forward. Cement fly-ash gravel pile is widely used in foundation treatment projects because of their high bearing capacity, low settlement, and low cost. The article adopts FLAC3D to conduct a numerical simulation study of the deformation characteristics of cement fly-ash gravel pile, the results show that cement fly-ash grave pile composite foundation produces vertical displacement with obvious zoning phenomenon under the action of upper load, horizontal displacement mainly occurs in the area of soil between piles, and the soil is extruded dense. The displacement and separation phenomenon of the interface of cushion and foundation are significantly different in the position of the pile and the position of soil between piles. The deformation of the CFG pile composite foundation is influenced seriously by the cushion as the weak zone of the CFG pile. The findings provide a reference and basis for the design and application of cement fly-ash gravel piles.