The invention belongs to the technical field of operating maintenance of power stations, particularly relates to a PRATT large-caliber butterfly valve seat seal ring replacement method and aims at researching and developing of the PRATT large-caliber butterfly valve seat seal ring replacement technology. By means of the PRATT large-caliber butterfly valve seat seal ring replacement method, the problem that this type of large-caliber butterfly valve equipment of Qinshan Third Nuclear Power Company Ltd. cannot be repaired is solved; the spare part purchasing and on-site repair cost is saved, andthe requirements for field work are met. The method is characterized by comprising the steps of cleaning an old valve seat, assembling a valve seat, conducting air testing before glue injection, conducting glue injection and conducting cleaning and checking. Since the PRATT butterfly valve seat seal ring maintenance and replacement technology is developed successfully, about 130 seawater system large-caliber butterfly valves are sequentially repaired successively in total, and the equipment purchase funds are saved by RMB 50 million or more.