The invention discloses a PBL stiffening type flat steel box concrete plate arch and a constructing method thereof. The PBL stiffening type flat steel box concrete plate arch comprises a main arch which is assembled by at least one arch section. Each arch section comprises a rectangular steel box, a plurality of PBL stiffening ribs are arranged at a top plate and a bottom plate of the steel box and the inner wall of a side web plate in the axial direction of the arch, and the steel box is internally filled with concrete. The problems that masonry and a reinforced concrete plate arch in the prior art is complex in construction process and great in retraction stress in the arch are solved. The PBL stiffening type flat steel box concrete plate arch is simple in structure, reasonable in design, convenient to construct, excellent in mechanical property, and construction quality is easy to control.