The invention discloses a construction method of a highway bridge pile foundation of a steep slope region in a mountainous area. The construction method comprises the steps that after the construction position of the pile foundation is determined, a slope face is reinforced through mesh hanging anchor rods on the slope face of a steep slope first of all; then, cement mortar layers and steel reinforcing meshes are additionally arranged layer by layer to reach the required depth of the pile foundation; then, reinforcing is conducted through an anchoring device of a special structure, pile hole drilling is conducted, steel casings with mortar leakage holes are placed, and steel reinforcement cages are placed in the steel casings; and finally, piles are formed through pouring, a bearing platform is arranged on the piles, and then pile foundation construction is completed. The construction method is easy and convenient, and construction is easy; and due to the arraignment of the specific structures of the anchor rods, the anchoring device and the like, the construction quality is further guaranteed.