Abstract: The cement-soil column, as one of the commonly-used ground improvement methods, can effectively improve the bearing capacity of foundation and reduce settlement. There have been many researches focused on this topic. However, the stress state of the composite soil is not clear in field and model tests, therefore, its real mechanical behavior is difficult to be obtained. A series of large-scale triaxial tests are conducted to investigate the unit cell of the composite soil with cement-soil columns under static loading. The influences of replacement ratio and confining stress on the peak and residual strengths are gained. An empirical method for calculating the equivalent cohesion of the composite soil is proposed, providing a reference for the design of composite foundation with cement-soil columns.