The invention discloses an anchoring device adapting to structural longitudinal movement and a mounting method, and relates to the technical field of bridge anchoring. The anchoring device comprises acable guide pipe, wherein an inhaul cable is arranged in the cable guide pipe in a penetrating manner and is fixedly connected with one end of the cable guide pipe through an inhaul cable sealing barrel; a horn-shaped steel casing which is located in the cable guide pipe, wherein the inner diameter of the horn-shaped steel casing is gradually increased in a direction away from the inhaul cable sealing barrel, and the outer wall of the horn-shaped steel casing is fixedly connected with the inner wall of the cable guide pipe. When the inhaul cable longitudinally deviates along with a main beam,the outer wall of the inhaul cable is tightly attached to the inner wall of the horn-shaped steel casing, the inhaul cable is limited to deviate from the end opening of the inhaul cable sealing barrel, and the concentrated cutting force of the inhaul cable and the end opening of the inhaul cable sealing barrel is transferred to the horn-shaped steel casing, so that the situation that the larger concentrated cutting force is applied on the inhaul cable locally is avoided, and the safety of an anchoring system is improved.