The invention relates to a method using a crossing cable crane to erect a main truss of a suspension bridge in a mountain area in the way of retrogression assembly. The method comprises the following steps: 1. assembly fields for steel truss girder member bars are arranged in front of tunnel portals under tower legs on both sides of the suspension bridge, first sections of truss girders are simultaneously assembled on pedestals on both sides, and hanger rods are installed; 2. side truss is assembled; 3. middle truss pieces are assembled; and 4. the steps are repeated until the assembly of themain truss is accomplished. The method utilizes the crossing cable crane as an aerial mobile travelling crane to realize the retrogression assembly of the truss girders from a bridge abutment to the midspan by a retrogression method; and as hoisting equipment is not arranged at the well assembled truss girders but at a main cable with high bearing force, the deflection of a suspension arm is greatly reduced, and the complexity of the linear control of the truss girders is changed. The method has the advantages of high safety and construction speed.