HT5SS]In this paper, first, the basic theories of calculating stress intensity factor at a crack\|tip by FEM are provided. Then, the stress intensity factors are calculated about thick\|walled cylinders with various crack size internal semi\|elliptical surface crack under autofrettage residual stresses and its change laws are studied. These researches in this paper are very valuable to design, making and fatigue analysis about thick\|walled cylinder structure.