The invention discloses a construction method for advanced reserving of side soil for a middle drift of a highway large-cross-section tunnel. The construction method is suitable for construction of the highway large-cross-section tunnel with the length being 12m or above and tunnels at grade III and grade IV surrounding rock sections with certain self-stabilizing conditions. According to the construction method, after staking out in survey, during the tunnel excavation process, three steps including an upper step, a middle step and a lower step are formed, and a plurality of different positions in the front and the back are excavated in a mutually staggered mode, wherein rock and soil in the middle portion of the upper step and rock and soil in the middle portion of the middle step are excavated at the same time. The middle step reserves side soil, all branch portions are supported in time to form a whole support, and the operation cycle time is shortened. All the portions are excavated and supported by adopting a tunnel construction method that the portions are staggered in the longitudinal direction of the tunnel and are advanced in parallel. The construction method for advancedreserving of side soil for the middle drift of the highway large-cross-section tunnel innovatively improves an existing construction method and is matched with corresponding equipment improvement measures, so that the rapid construction potential can be better exerted, and the purpose of safe, reliable and rapid construction is achieved.