Abstract: The pressure tube is a necessary accessory for the phase modifier for normal service, and which is usually connected by welding, therefore, the welding quality directly affects the safety of the phase modifier. In this paper, we calculated and analyzed the stress intensity factor of both the pressure pipe with incomplete penetration defects and its tip crack by using the Comsol simulation software. In addition, we evaluated on the safety of welded tubes based on the standard GB/T 19624-2019 "Safety Assessment of Pressure Vessels with Defects in Use". The results show that the critical dimension of the depth of incomplete penetration defect is 0.85 mm, and the critical value of the safety evaluation index is 0.44. The crack intensity factor at the tip of the incomplete penetration defect is greater than the threshold stress intensity factor of the tube material (1.98 MPa ·mm^1/2). The crack expands under the action of external load, and the stress field intensity factor K_I at the crack tip increases. When the stress field intensity factor at the crack tip reaches the fracture toughness of 20 steel 30.4 MPa ·mm^1/2, the crack will expand unsteadily, thus resulting in material fracture.