Tunnel portal structure and construction method

The invention relates to a tunnel portal structure which is arranged on a mountain and close to a slope of the mountain. The tunnel portal structure is characterized by comprising an expanded foundation, a bias wall and an arch frame, the enlarged foundation is fixedly arranged at the side slope of the mountain body, and a supporting surface is formed at the top of the enlarged foundation; the arch frame is of a hollow closed structure, the top of the arch frame is in an arch shape, part of the bottom of the arch frame is buried in the expanded foundation, the rest part of the bottom of the arch frame is arranged at the top of a corresponding mountain body, a bottom plate is formed at the rest part of the bottom of the arch frame in a pouring mode, and the top face of the bottom plate is flush with the top face of the expanded foundation; the bias wall is arranged on the top of the enlarged foundation and close to the side slope of the mountain, and the bias wall is connected with the corresponding side porti