Abstract: This paper analyzed the optimum autofrettage pressure of a thick wall cylinder under the in-ternal pressure and mode I stress intensity factor（SIF） for axial semi-elliptical cracks on the surface of the autofrettaged thick wall cylinder based on ANSYS.The SIF along the crack front are directly computed by 3D finite element method for a wide range of variations of the crack geometry.Also cracks on both surfaces of the autofrettaged thick wall cylinder were considered.The results show that the optimum autofrettage pressure of cylinder changes with the internal pressure,the SIF of the axial crack on the inner suface decrease after autofrettage,and the differences decrease while the crack depth increases,the axial crack outside the cylinder causes a decrease of about 1% of the SIF of the crack inside.