The threeline transition section at exit of Xinkaotang Tunnel on GanzhouLongyan Railway is located in waterrich fully weathered granite stratum; and it has many characteristics, i.e. large excavation span (30.3 m), shallow cover and large excavation area (396 m2). The structure selection and construction method for shallowcovered largespan tunnel on threeline transition section is studied; and subsection design method for different crosssection, which suits for transition section of tunnel, is put forward. The transition section (215 m long) is divided into 6 subsections. The widening values are 0.8 m, 2 m, 4 m, 6 m, 8 m and 10.3 m respectively, with area above rail varying from 85.16 m2 to 200.02 m2. The subsection widened by 0.8 m is constructed just like normal doubleline tunnel, while subsections widened by 2 m, 4 m, 6 m are constructed by CRD method, doublesidewall heading method, and compound doublesidewall heading method with dilated wall respectively. The construction method of “dilated wall + enhanced arch” is put forward for subsections widened by 8 m and 10.3 m. The construction practice of Xinkaotang Tunnel shows that the design of construction methods and the corresponding support structure are feasible and rational. The results can provide reference for similar projects in the future.