The invention relates to the field of bridge construction, in particular to a long-span cable stayed bridge steel truss girder mounting method. By using a girder erection crane to completely hoist bridge deck slabs and third quarters of steel truss girders, a construction platform for assembling other rods can be established as early as possible, aloft operations are reduced, cantilever erection time is shortened, construction environment is improved, and construction efficiency is improved. In addition, only the bridge deck slabs and third quarters of steel truss girders are hoisted, and accordingly hoisting weight is reduced, and investment on the girder erection crane is saved.