Abstract: Internally supported diaphragm wall is a commonly used support for deep foundation pits. The lateral deformation control of diaphragm wall in foundation pit excavation is important for foundation pit design and construction. In order to predict the lateral deformation of diaphragm wall effectively and accurately, this paper establishes the deformation calculation model of diaphragm wall based on a generic third-order plate theory by considering the joint action of soil and support on both sides of the wall, and establishes a solution system of lateral deformation of the wall by combining with the Pb-2 Rayleigh-Ritz method and the principle of minimum potential energy. The lateral deformation of the diaphragm wall in the process of excavation is obtained. In order to verify the proposed model, an indoor model test was carried out to simulate the excavation of the foundation pit. The deformation development of diaphragm walls with different thicknesses in each stage of excavation was recorded. Comparison was made between the theoretical prediction and the measured data, which reveals that the proposed model can capture the deformation development of the diaphragm wall well. The proposed model can comprehensively consider excavation depths and support arrangements to obtain the lateral deformation of diaphragm wall, hence provides technical support for the optimization of the design and construction of foundation pit support structure.