Ribbed supporting structure is a new type of foundation pit supporting structure, which is composed of steel sheet piles and rib plates that rigidly connected with the piles. The existing engineering cases have proved that the supporting performance of the structure is satisfactory. However, the supporting mechanism of this structure is yet to be further investigated and proved by experiments. In the process of foundation pit excavation, the deformation and mechanical characteristics of the ribbed support structure and the stability of foundation pit are studied by finite element method, and the calculated results are compared with the measured values. Moreover, the influence of rib-width and rib-spacing on the horizontal displacement and stability of the structure is analyzed. According to the analysis, the maximum horizontal displacement prediction formula is proposed, which can predict the actual possible displacement of the support structure. The results showed that the displacement and internal force of the ribbed support structure are much smaller than those of the conventional support structure of steel sheet piles for the same project, for example, the maximum horizontal displacement of the sheet piles is decreased by 71.56% when the rib-width is 1.60 m and the rib-spacing is 0.80 m. In this case, the bending moment distribution of sheet piles is relocated, the bending moment of the sheet pile is greatly reduced, so the stability of the foundation pit is improved. Rib plates can not only greatly improve the bending stiffness of the structure, but also can produce a larger anchor tension for the structure to resist the soil pressure on the front row of steel sheet piles.