The invention provides a large-span concave fish belly type double-slope roof truss and an installation method thereof. The large-span concave fish belly type double-slope roof truss comprises a lighttruss area, reinforced truss units and overhanging truss units. The light truss area is arranged at the middle position of the roof truss and comprises a plurality of concave fish belly type light truss units arranged parallelly, the reinforced truss units are longitudinally arranged on two sides of the light truss area, and the overhanging truss units are arranged on the periphery of the roof truss. Latticed columns are used as a tubular stand column type, a main truss lower chord is molded into an arch-shaped main truss, a lower chord rod is in an arch shape, secondary trusses are monolithic trusses, the main truss and the secondary trusses are distributed reasonably, and the strength is guaranteed. Compared with a general fish belly type truss, the large-span concave fish belly type double-slope roof truss has a close structure and an opposite molding, on the basis of the common main truss, the secondary trusses are connected crosswise through upper and lower connection rods, an integrated fish belly type double-slope roof truss structure is formed, two ends are thick and heavy, and the middle is thin and light. The concave fish belly molding of the whole truss can be controlled according to sizes and lengths of triangular trusses at the two ends of the secondary trusses and middle monochords.