Fifth Yangtze River Bridge in Luzhou is an extra-long span half-through steel box arch bridge with thrust. The main girder deck system is subject to the direct action of vehicles, pedestrians, rail transit and other loads. At the same time, it produces a huge horizontal thrust to the arch base foundation through the slings and arch ribs. Therefore, the selection of the main girder structural type is directly related to the comfort of traveling, the steel consumption economy and the arch base foundation engineering volume. The three main girder schemes are compared from the structural form of the rail transit, the stiffness demand, the deformation limit and the force performance. The two-side box type flat steel box girder is finally selected as the deck system.