The hydraulic bulging strengthening technology is a kind of strengthening technology of large generator’s retaining ring. In the right punch working angle and sealing pressure of equipment, the ring billet is deformed uniformly by high pressure water. The experiment in 5MN oil-hydraulic machine proves that the ring billet is deformed uniformly while the punch working angle is 60° and the pressure between the punch and the rings is 1MN. When the internal and external diameter deformation of the ring billet is 8.5% and 14.0%, the external generatrices of the ring are nearly line. The ring billet is 1/5 of 300MW18-18 steel retaining ring. It may improve productivity to measure real-time thickness of the ring billet with ultrasound sensors and control pressure of the oil-hydraulic machine and ultra-high pressure pump with 80c51.