In SSCC floor systems the concrete slab is composite with both steel stringers and crossbeams or only with stringers. The horizontal bending of the crossbeams of SSCC floor systems is not ignorable. From the deformation conforming condition, theoretical formulas for SSCC floor systems of double-railway bridges with two stringers are developed. The factors which influence the horizontal bending of crossbeams are discussed. Some conclusions are obtained. When the width of the SSCC floor system is given, the main factors which influence the horizontal bending of the crossbeams are the axial stress of the lower chords, the distance between the side stringers and the lower chords, and the continuous length of SSCC floor system. Increasing the horizontal inertia moment of crossbeams almost cannot reduce the horizontal bending stress of crossbeams. A slight horizontal rotation of the lower joints such as 10-4 rad can release more than 3% crossbeam end horizontal bending moment.