An on-site installing method foran anchor tension plate of a steel structure cable-stayed bridge is provided. A steel box girder and an anchor tension plate are manufactured separately, When steel boxgirder is pre-assembled, marking the installation control point and positioning datum line of the anchor plate on the steel box girder, after the installation of the steel box girder is completed, according to the design and installation angle of stay cables, the control points and positioning datum lines of all anchor plates on the steel box girder are re-measured. when the anchor plates are installed on the site, the center line of anchor plate bottom plate coincides with the positioning datum line on the steel box girder, and the axis extension line of anchor plate cable conduit coincideswith the intersection point of the top surface of the steel box girder and the installation control points. At that upper end of the anchor plate, a perpendicular line perpendicular to the orthogonalprojection line of the cable duct axis is arrange, and the angle of the anchor plate is adjusted until the perpendicular line point to the positioning datum line of the top surface of the steel box girder, so as to realize the positioning of the anchor plate, and the bottom plate of the anchor plate and the top surface of the steel box girder are welded and fixed at the same time.