The invention relates to a tunnel excavation construction method; an excavated tunnel comprises a first main line tunnel conventional segment, a first main line tunnel expansion segment, a second main line tunnel conventional segment, a second main line tunnel expansion section and a maximum cross section tunnel section; the method comprises the following steps of: firstly excavating the maximum cross section tunnel section, and then excavating in a transitional way from a big cross section to a smaller cross section of the maximum cross section tunnel segment, thereby realizing the excavation construction on a subway interval tunnel containing a junction line and a stop line in a big to small necked-down section mode. The method has the beneficial technical effects: (1) only one verticalshaft needs to be arranged, and the construction workload is less; (2) the overall process is in a transitional way from the big cross section to the smaller cross section, and the reverse expansion construction is not needed so as to avoid the risk of expansion excavation construction; (3) the excavation construction is carried out on the subway interval tunnel containing the junction line and the stop line in a big to small necked-down section mode, a roundabout pilot tunnel method is avoided from being used, the working procedures are simple, and the temporary first-lining workload is small.