The invention discloses an excavating and supporting method for a large-section tunnel delta-shaped functional structure. According to the size of a tunnel, an excavating area is divided into a left area, a middle area and a right area, each area is divided into a plurality of guide holes from top to bottom, all the holes are excavated according to the construction mode of left-right symmetry fromtop to bottom and middle at last, the tops, bottoms and side faces of all the guide holes are supported during excavating, then a closed loop is formed, part of temporary steel supports in supportingare detached after one cycle ends and surrounding rock is stable, and the rest of temporary steel supports are reinforced to become permanent steel supports. By improving a dual-side-wall pit guide method, the tunnel excavation temporary supporting is converted into permanent supporting. The construction cost can be effectively saved, a certain construction cycle can be shortened, and the construction safety coefficient can be increased.