Calculating stress intensity factor is an important task of fracture mechanics, specially in the thick walled cylinder with inner pressure, which has macro crack. The value of KІ is crucial for safety life. The software ANSYS was used to calculate the stress intensity factors of the walled cylinder in this paper. Crack along the axis in the inner face of the tube can be predigested a long plate with a center crack. According to the symmetry of this model, the calculated model was built. The results show that the relative error is under 5% and the method is available to satisfy the precision engineering requirements. In addition, the error and how to reduce it were analyzed.