To prevent safety accidents and improve the safety of in-service storage tanks with defects, the stress intensity factors of cracks of different sizes and locations in tanks with crack defects were studied. Taking the storage tank with a semi-elliptic surface crack under internal pressure as the research object, the influence of different crack length c, the ratio of crack depth to tank thickness (a/t), and the location of the crack on the maximum value K_Ⅰ,max of the stress intensity factor at the crack tip was studied by using the interaction integration method. The results show that when the crack is located in the cylinder, K_Ⅰ,max> increases with the increase of crack length and the ratio of crack depth to tank thickness, and the growth rate is not fixed; When the crack is located in the elliptical head, K_Ⅰ,max decreases with the increase of the angle θ between the geometric center of the crack surface and the connecting line of the elliptical head center and the long axis of the elliptical head, and increases with the increase of the crack length; When the crack is located in the high stress area near the opening, the change rule of K_Ⅰ,max with the crack length is consistent with that of the cylinder; When the crack length is greater than the crack depth, K_Ⅰ,max appears in the middle of the crack leading edge, and vice versa. The research results have a certain reference significance for the safety evaluation and inspection rules of in-service defective storage tanks and other pressure vessels.