Based on the transformation project of rain and sewage diversion in a city, the optimized design of Larsen steel sheet pile supporting structure in pipe-pit excavation was made. Using three-dimensional finite element method to analyze the retaining characteristics of Larsen steel sheet pile, soil deformation of adjacent transverse brace in the middle position and surface subsidence were regarded as design control indicators; through numerical analysis of the geological conditions, load, pile length, horizontal interval of transverse brace and other factors on the sensitivity of the design control indicators, horizontal displacement of brace was regarded as a key factor in the optimal design, and accordingly, related optimized program was proposed. The implementation results show that the optimized scheme can improve the efficiency of construction, protect the safety of construction, and provide a reference for the design and construction of similar projects.