The traditional support structure of double-row piles single portal frame is a frequently used supporting design for foundation pit in soft soil area because of the advantages of simple construction without inner support, small investment and space saving. But for super-large foundation pit of medium excavating depth in soft soil, large lateral displacement and settlement as well as poor structure stability often appear. Combined with the improvement on the traditional support structure of portal frame, double portal frame support structure without inner support was proposed in a large medium-depth excavation project in soft soil to further improve the overall safety and stability and control the lateral displacement of the supporting structure. The applicability and feasibility of this improved design are verified through a basement foundation pit supporting project in Shaoxing County, which provides valuable experience for the similar projects in the future.