The invention discloses a horizontal tunnel portal protection structure and a construction method thereof. The structure comprises concrete around a tunnel portal, horizontal limiting sleeves, main ribs, cover plate wooden stepping bars and a cover plate, wherein the tunnel portal is a rectangular portal; the horizontal limiting sleeves are vertical to two long edges of the rectangular tunnel portal, and are embedded into the surrounding concrete in an equal-height spacing manner; the horizontal limiting sleeves are grouped in a manner that openings of every two orifices are opposite along the two long edges of the rectangular tunnel portal; the number of the main ribs is equal to the number of groups of the horizontal limiting sleeve; two ends of each main rib penetrate into every group of two horizontal limiting sleeves; at least two cover plate wooden stepping bars are vertical to the main ribs and are placed on the main ribs; the cover plate covers the cover plate wooden stepping bars to close the tunnel portal; the upper surface of the cover plate is flush with the upper surface of the surrounding concrete. The horizontal tunnel portal protection structure is easy and convenient to construct; the construction time can be shortened, and the construction work above cannot be affected; the material cost is greatly saved; the horizontal tunnel portal protection structure is more attractive and safe and is general.