Pile-soil stress ratio is an important index in the research of composite foundation. Currently, the studies regarding the pile-soil stress ratio mainly focus on the general composite foundations, such as mixing pile and gravel pile. However, the theoretical calculation of stress concentration ratio of stiffened deep mixed (SDM) pile has been rarely reported. From the deformation modes of soil, cement and the composite pile element, the negative friction resistance of pile, pile cap and pile tip penetration to bearing layer is considered comprehensively, the compression deformations of soil around the pile, cement soil pile and composite pile body are analyzed, and a stress ratio calculation formulation is derived to describe the relationships among stiffened deep mixed composite foundation piles, cement soil pile and soil. Further, the effects of core pile diameter of stiffened deep mixed pile, core pile length and soil cement pile length on the pile-soil stress ratio are analyzed. It is found that the pile soil ratio increases with increasing the core pile diameter, length of stiffened deep mixed pile and pile length of cement soil. By comparing the computed results with the field data from a field test, the computed results and measured data agreed well and the reliability and feasibility of the proposed method was verified, which can be beneficial to guiding the actual practice.