The invention discloses a convoluted excavation method for an undermined tunnel with an extra-large cross section, which comprises the following steps of: a, excavating a right side small pilot tunnel; b, excavating a transverse channel; c, excavating a left side horizontal upper pilot tunnel; d, performing multistep expanding excavation on a middle lower pilot tunnel; e, excavating an inverted arch and core soil; and f, constructing a secondary lining of the large cross section of the tunnel. Compared with the conventional tunnel excavation method, the convoluted excavation method for the undermined tunnel with the extra-large cross section has the advantages that: the small pilot tunnel climbs and is divided into a plurality of parts to be excavated when a small cross section is abruptly changed into the large cross section, so that the safety risk of excavating the large cross section at one time is reduced, and the problem of traffic organization in the tunnel subjected to single-head advancing is solved by a circuitous folding mode; back break during excavation is effectively controlled, the using amount of various materials is saved, and the time consumption and the wastage rate of mechanical equipment are reduced; and compared with the conventional center diaphragm (CD) and double-side-wall excavation methods, the method is easy to operate, good in cross section excavation effect, quick in excavation progress and high in safety performance.