The invention relates to a single-arch large-span shallow-buried excavation tunnel reinforcing system and a construction method thereof. During a single-arch large-span shallow-buried excavation tunnel construction process, risks such as deforming and collapsing exist, and reinforcing processing is needed. The single-arch large-span shallow-buried excavation tunnel reinforcing system is a supporting frame structure in a tunnel preliminary bracing part and comprises a combination steel pipe concrete column at a tunnel center line, the bottom end of the combination steel pipe concrete column is fixed at a column foundation, the top end of the combination steel pipe concrete column is fixed at the preliminary bracing part, bottom plates are arranged on the two sides of the column foundation, vertical supports are fixed above the bottom plates, the vertical supports and one end of an oblique support are fixed to the preliminary bracing part, one end of a horizontal support is fixed to the preliminary bracing part, and the other end of the horizontal support is fixed to the combination steel pipe concrete column and is firmly connected to the vertical supports. The system and method are used for reinforcing processing on a single-arch large-span shallow-buried excavation tunnel where accidents such as large deforming, collapsing and water bursting happen or are about to happen, the advantages of a door-shaped support and a sector support are combined, large span is changed into small span, and deforming caused by construction, and deforming caused by construction can be controlled effectively and quickly.