Method for constructing extra-large variable cross section tunnel

Disclosed is a method for constructing an extra-large variable section tunnel. The method comprises: excavating a left pilot tunnel (11) and timely constructing primary supports and temporary supports; packing large boot-shaped wall feet (13) and constructing a right pilot tunnel (12) by the same process; excavating the upper half part of the left side of a main tunnel (2) and timely constructing a first-layer primary support (23), a temporary vertical brace (91) and a temporary inverted arch; excavating the lower half part of the left side of the main tunnel (2) and timely constructing the first-layer primary support (23) and the temporary vertical brace (91 ); constructing the upper half part of the right side of the main tunnel by the same process, and excavating a middle arch part and timely constructing the first-layer primary support; constructing a second-layer primary support (24) and removing the temporary vertical braces and the temporary inverted arches of the arch part; excavating a central part and timely constructing primary support (8) of the inverted arches; removing the temporary supports (17) of the left and right lower pilot tunnels, packing an inverted arch secondary lining, and filling the bottom of the tunnel; and finally one-time packing an arch wall secondary lining (22) by means of a model board trolley. The construction method effectively reduces the safety risk of removing the temporary supports, increases the operation freedom degree and accelerates the construction schedule.