Based on the bi-level, the load transfer mechanism of stiffened piles with long core pile was analyzed. The“cement-soil plug”was formed when a PHC pipe pile was implanted into bi-direction cement mixing pile in a concentric manner. The theory of cavity expansion was adopted for analyzing the disturbance on the side soil, and the radial stresses of pile-soil contact surface before and after the complete occlusion of cement soil plug were calculated. Considering the shear strength, the calculation equation for side friction was reduced then. On the basis of the engineering, side friction forces were calculated by equations in this paper and the code respectively.Comparing with those calculated based on the code, the forces calculated by the equation proposed in this paper coincided better in mucky soils and lacked in the silty clay. Affected by the load and data acquisition mode, forces measured in-situ were less than the calculated ones.