The invention provides downhole rail mounting dies, belongs to the technical field of coal mining equipment, and particularly relates to auxiliary rail mounting dies. The dies aim to assist in rail mounting and laying, so that rails can be laid easily and conveniently, and the track gauge and the rail parallelism are made to meet the quality standard. According to the technical scheme, a supporting device and the dies arranged on the supporting device are included, the supporting device comprises a supporting plate and a height adjusting device, the supporting plate is provided with two sets of dies, and the two sets of dies are arranged on the two rails respectively. According to the downhole rail mounting dies, the domestic blank in rail mounting is made up for, the dies can be used for mounting and laying the rails on or below a downhole of a coal mine, the labor intensity of workers is greatly lowered, the rail laying quality requirement is further guaranteed, and time and labor are saved.