Abstract: The author has presented a hydraulic expansion device of reducing load and put forward the principle of plastic forming, the calculation formulae of designing the devices and the qualitative analysis of the process establishing the internal pressure(1)(2)(3).In cooperation with the units concerned, the author has revealed the inherent forming laws of the hydraulically expanded retaining rings by reducing load method after a series of simulated tests. The laws have directly guidable significance on the theory and Practice for manufacturing 300000 KW retainig rings(4)and the larger one. Some of the the oritical studies and experimental verifications are valuable for this field.