Based on the analysis of deformation mechanism of the composite foundation with deep mixing piles, a calculation pattern of the composite foundation was firstly presented according to the unit cell model. Then considering the impact of the distribution behavior of side friction between the pile and the surrounding soil and the effective pile length, the calculation method for the displacement of pile in the treated area was proposed by elastic theory. Thirdly, the settlement calculation method of the underlying stratum was suggested with the solution of Mindlin and Boussinesq.