Performance of stone columns in composite foundation was investigated through finite difference method(FDM) that was verified by published experimental results of model tests, and then the effect of embedded depth of composite foundation on failure mode of single column and group columns in saturated clay soil was analyzed. The research results show that with the increase of the embedded depth of composite foundation, the surrounding pressure of the single column increases, the lateral constrains function is enhanced, the horizontal maximum deformation along stone columns reduces, and the failure location move down gradually along stone column. In the cases of a group of stone columns, the failure mode of columns is determined by the location of columns under composite foundation. Compared with side stone columns whose failure location is near to the top of the columns, the failure of the central column is located in the deeper parts of the columns. The lateral restraint effect of the foundation embedded depth on the side column is obvious. The failure mechanism of the friction columns transforms from the initial bulging(shearing)along the horizonal direction to downward penetration when the embedment depth becomes large.