Abstract: Pile length is a key parameter in cement mixing pile composite foundation design.The pile length must be limited within the effective length of cement mixing piles.The effective length of piles not only depends on soil and pile characteristic,but also depends on grouped piles and loads distribution.The pile stiffness is reduced by the pile-pile interaction and the effective length of piles becomes longer.A new optimum design method of cement mixing pile length is to control the composite foundation settlement to get a desired numerical value.