In order to investigate the load transfer behaviors of the equal-core stiffened deep mixed pile under a flexible foundation， the stress-deformation modes of this kind of pile subjected to vertical load are analyzed first. Based on some experimental results， the double-exponential function load transfer model is introduced to characterize the interface shear softening process between the core pile and cemented soil. And the ideal elastic-plastic load transfer model is introduced to describe the interface shear deformation properties between cemented soil and the surrounding soil. The pile is then divided into several elements. Based on the load transfer method， a load transfer analysis model is established for the core pile， the cement soil pile， and the surrounding soil. And the recursive formulas used to calculate the stress-deformation behaviors of each element of the pile are further derived. A calculation flow for obtaining the distributions of axial force， side friction and interface relative displacement of the core pile， the cement-soil pile， and the surrounding soil is established by using the iterative calculation method. An analysis and calculation method for the load transfer behaviors of the equal-core stiffened deep mixed pile under a flexible foundation is thus proposed. Finally， the reliability of the proposed theoretical method is verified by comparing the calculations with the results of the field test and engineering case. At the same time， based on the calculation results， the distributions of the side friction and axial force of the equal-core stiffened deep mixed pile under a flexible foundation are discussed and obtained. The research achievements can provide guidance for the design practice of equal-core stiffened deep mixed piles under a flexible foundation.