The composite foundation with capped rigid piles and geo-synthetics is a new technology. A plane of equal settlement exists in the fill through soil arching. The horizontal membrane can restrain the lateral deformation of the pile and subsoil. Most loads can be effectively shifted to the piles and the deep layer. The total settlement and differential settlement can be controlled. The technology is particularly fit for soft soil ground treatment engineering. The mechanism of interaction among fill, foundation soil, geo-synthetics and pile (caps) was analyzed. A theoretic design method of the composite foundation with capped piles and geo-synthetics has been presented and a project case was cited in the paper. This study will improve the design theory of ground treatment.