The FEM method and theory of structure mechanics were adopted to analyze the relationship between the shape or the size of the opening and the support system stiffness of the beam and slab structure. The response of the lateral displacement and inner force of diaphragm wall with the variation of support stiffness in different depth were studied. The results show that the relation curves of the structure stiffness and opening size are different between the circular-shaped opening and square-shaped opening; however, the relation curves of the structure stiffness and opening area are similar between different opening shapes, and the opening area ratio can be used to determine the minimum structure stiffness and the maximum opening size of the beam and slab structure in engineering design. The lateral displacement of enclosure structure increases with the decrease of the support system stiffness, but no adverse impacts are noted to the inner force of the enclose structure. The inner force of the enclosure structure can be optimized by adjusting the opening size and shape of the beam and slab structure.