Tianxingzhou cable-stayed bridge firstly adopts the innovative structure which is composed of three main trusses and three cable planes, and its mechanical properties are different from those of the traditional structure with double main trusses and double main planes. In this paper, based on the study about the cable tension distributed properties of the completed bridge with three cable planes, and through comparing analysis on the bridge with three main trusses and that with double main trusses, the mechanical properties of the bridge with three main trusses and three main planes were investigated. In the end, the calculated results show as follows: cable tension distributes well after rational optimal calculation. Although there will be more construction works for the added cable plane, per cable tension reduces, and it makes the anchor zone with simpler structure and cable replacing more conveniently; Under same level stress, the three-main-truss structure has smaller component, which makes construction more conveniently; The three-main-truss structure has a lower torsional stiffness, but it can be effectively improved by enhancing axial rigidity of lateral bracing; Under the condition of the same materials, the bridge with three main trusses has lower stress. Therefore, the cable-stayed bridge with three main trusses and three main planes is the rational choice for multi-line railway and rail-cum-road bridges.