A parametric model of the pressure vessel's transition zone was set up,using APDL language by conventional finite element software ANSYS.On the basis of analysing the distribution of transition zone's stress intensity and taking advantage of the optimization design module of ANSYS,the dimensions of the transition zone were optimized.The relationship between the maximum stress intensity and the dimensions and a best design of the transition zone were prouided.The result shows that the maximum stress intensity in the best design can be reduced by 10.6% than before.