As a new technique of excavation support structures, outrigger-type diaphragm wall has got an application in deep pit engineering, though the design of its parameters still lacks theoretical guidance. Based on a large-scale deep pit case, a 3-D finite element model is presented to simulate the behavior of outrigger-type diaphragm wall using the software ABAQUS. The soil is assumed to behave as a modified Cam-clay model. The effect of the outrigger length on deformation and internal force of the supporting structure is studied. It is found that the basal heave ,the ground settlement and the horizontal displacements of the retaining wall will increase with the increase of the outrigger length, when the bottom elevation of the supporting structure keeps unchanged. When the outrigger length goes beyond a certain length, the rate of increasing becomes larger. In the outrigger-type diaphragm wall, the vertical stress of inside and outside wall approaches the maximum in the vicinity of the variable cross-section. At the same time, the variable cross-section produces a shear stress mutation because of the abrupt change of stiffness. There exists a reasonable length of the outrigger, which could meet not only the technical requirements of the supporting structure, but also the needs of saving the cost and reducing the difficulty during the construction.