The invention discloses a flat-layer girder section structure of a combined highway and railway bridge. The flat-layer girder section structure comprises a railway girder. The two sides of the railwaygirder are connected with a left highway girder and a right highway girder. The top face of the railway girder and the top faces of the left highway girder and the right highway girder are located onthe same plane. Anchoring systems are arranged on highway and railway parallel separation zones between the railway girder and the left highway girder and between the railway girder and the right highway girder. The anchoring systems are used for connecting stay cables. Each of the railway girder, the left highway girder and the right highway girder comprises multiple diaphragm plates, and the section of each diaphragm plate is of a section thickened structure. By the application of the girder section structure, the stay cables are connected to the two sides of the railway girder instead of the two sides of a whole highway and railway girder, the interval between the stay cables on the two sides is reduced, the girder bending moment is reduced, girder transverse stress is more reasonable,the diaphragm plates of the section thickened structures are used by the girder, the stress condition of the girder section structure is improved, the interval between the oppositely-arranged diaphragm plates can reach 4 meters or can be over 4 meters, compared with existing design of the same structural strength, the steel use amount is reduced, and cost is greatly reduced.