The foundation stiffness values in transverse design and calculation of immersed tunnel structure are very important. As a result, the variation law of foundation stiffness with stress is studied by theoretical analysis; and then the approximate analytical solution of the distribution of transverse foundation stiffness of immersed tunnel is derived based on constitutive theory of hardening soil model. The study results show that: (1) The distribution of transverse foundation stiffness is related to the initial foundation stiffness, structural stiffness, foundation stress level and upper loads. (2) In case of soft foundation, transverse foundation stiffness is distributed as Wshape; and in case of hard foundation, the structure may be locally separated from the foundation at midspan, and the foundation stiffness between separation sections is horizontally distributed. (3) The results of numerical examples show that the analytical solution of the foundation stiffness distribution is consistent with numerical results, so as to provide a basis for the calculation of immersed structure.