Abstract: This paper systematically introduces the elastoplastic analysis and autofrettage method of the pressure nder .It describes in detail the 12,100kg/cm2 hydraulic autofrettage test of a cylinder with its outside diameter 240 mm ,inside diameter 40-82 mm, and the height 362 mm. A cross –hole with diameter 4 mm is drilled ,then 9，925 kg/cm2 cyclic hydraulic pressure is applied .The result of the test is an expectant success .The behavior of the cylinder is satisfactory in its later usage . Conclusions are made as follows: 1. Using the hydraulic autofrettage technique ,the ultimate fatigue strength of the triaxial chamber cylinder ,which has the changeable cross section and is made by the super strengthened steel 37SiMnCrNiMoV,can be increased . 2. The hydraulic autofrettage technique can improve stress concentration of the cross-bore of the high pressure vessels.