The invention relates to the technical field of tunnels, and provides a tunnel portal section structure suitable for steep terrains and a construction method thereof.A steep rock-soil body is vertically excavated through an upward slope step by step, a
soil nailing wall is adopted to support the upward slope while the upward slope is excavated from top to bottom, a box
culvert main body, a stand column
assembly and a
joist assembly are constructed, and the tunnel portal section structure suitable for the steep terrains is constructed. And constructing portal walls and cover arches on the two sides of the tunnel by using the constructed adjacent span bridge, the box
culvert main body and the
joist assembly as a construction platform, constructing a
pipe shed on the basis of the cover arches, backfilling in the second backfilling space to form a second backfilling layer, constructing a shed tunnel assembly, constructing a tunnel hidden hole, and completing the construction of the tunnel portal section structure. According to the double-layer frame structure, the foundation of an upper shed tunnel assembly can be provided, a channel is provided for an existing low-grade road on the lower portion, the double-layer frame structure is used for an adjacent bridge and a bridge
abutment, meanwhile, the upper portion of the box
culvert is backfilled to the top face of the
joist, a construction platform can be provided for tunnel entering, the lateral soil pressure of a mountain can be resisted, and the construction safety is high.