<p>The calculation of lateral earth pressure in the active zone of the foundation pit support is an important aspect of foundation pit engineering. Based on the cantilever-type deformation mode of foundation pit support, this paper simulates the large deformation of foundation pit support and the evolution process of lateral earth pressure, analyzes the relationship between lateral earth pressure and large lateral deformation, studies the calculation expression for support lateral pressure-lateral deformation, and verifies the application effectiveness of the expression based on different stratigraphic conditions. The results indicate that under the engineering conditions of this study, during the cantilever-type large deformation process of the foundation pit support, the evolution law of support lateral earth pressure exhibits non-monotonicity characterized by a ‘first decrease and then increase’ pattern. Initially, it decreases with the development of the plastic zone and the exertion of soil self-stability, followed by a rebound due to the interconnection of slip surfaces and the deterioration of foundation pit stability. This paper takes the lateral deformation of the foundation pit support under the active ultimate state of soil as the critical value, and comprehensively uses the traditional trigonometric function lateral pressure calculation model and data fitting method to construct the calculation expression of lateral earth pressure during the cantilever-type large deformation process of the foundation pit support. Under four different stratigraphic conditions, the maximum differences in lateral pressure during the ascending section between the simulation and the expression calculation are merely 17.40&#xa0;kPa, 17.97&#xa0;kPa, 19.31&#xa0;kPa, and 19.93&#xa0;kPa, respectively, proving the accuracy of the calculation results from the expression during large deformation processes of the foundation pit support. This study provides a clearer practical understanding for engineering design.</p>