The invention discloses an automobile body and auxiliary frame installing structure. The automobile body and auxiliary frame installing structure comprises an automobile body, an auxiliary frame, an auxiliary frame connecting bolt and a nut, wherein the auxiliary frame connecting bolt penetrates through the automobile body and the auxiliary frame to be tightly connected with the nut in a screwed mode. The automobile body and auxiliary frame installing structure is characterized in that a groove is formed in the automobile body, an automobile body bolt hole is located in the middle of the groove, the installing structure is further provided with a reinforcing plate, the middle of the reinforcing plate is a concave part, the concave part is installed in the groove in a matched mode, the periphery of the reinforcing plate and the automobile body are welded together, a bolt hole is formed in the middle of the concave part of the reinforcing plate, and a bolt rotating preventing boss is arranged by the side of the bolt hole. Due to the arrangement of the reinforcing plate, strength of connecting and tightening portions of the automobile body and the auxiliary frame can be improved, bolt compressing force can be evenly dispersed within a large-area range, and the phenomenon that the automobile bodydeforms or is damaged partially because tightening force of the bolt is too large is avoided. Furthermore, because the bolt rotating preventing boss is arranged on the reinforcing plate, the auxiliary frame connecting bolt can be effectively prevented from rotating in the screwing process, assembly operation is simplified and assembly efficiency is improved.