The invention provides an online repairing device and method for a dry quenching high temperature metal expansion joint, and belongs to the technical field of coking equipment maintenance tools and methods. According to the technical scheme, two semicircular sealing clamping rings are welded to the upper ends of two semicircular sealing guide sleeves respectively, the two semicircular sealing clamping rings and the two semicircular sealing guide sleeves are connected into a whole through connecting flanges, an upper sealing check ring and carbonized fiber packing are placed between the circumferential inner walls of the sealing clamping rings and an upper end pipe of a high temperature metal expansion joint, two semicircular sealing bodies are located in the circumferential inner walls of the lower ends of the two semicircular sealing guide sleeves respectively, the carbonized fiber packing is placed between the circumferential inner walls of the sealing guide sleeves and the sealing bodies, and a lower sealing check ring and the carbonized fiber packing are placed between the sealing bodies and a lower end pipe of the high temperature metal expansion joint. According to the online repairing device and method for the dry quenching high temperature metal expansion joint, online overall repairing can be conducted on the high temperature metal expansion joint subjected to leakage, the sealing effect of the system under various high temperature pressure working conditions is guaranteed, various compensation requirements of an original expansion joint are met, and production and equipment losses caused by shutdown overhauling are avoided completely.