Abstract: Aiming at the delayed leakage problem in welded joints of transformer oil tanks, the finite element models of the oil tank body and T-joint were established. Based on the vibration load spectrum of transformer oil tanks, fatigue life analysis of the welded joints was conducted using nCode DesignLife software. The effects of defect-free welds, discontinuous welds, and internal unfusion on the fatigue life of the joints were studied. The results show that the fatigue life of defect-free T-joints is 102 years. For discontinuous weld defects, the greater the discontinuity distance, the shorter the fatigue life of the joint. When the distance reaches 1.00 mm, the fatigue life reaches the critical value of the specified service life of 30 years.The effect of the unfusion defect locations on fatigue life is that the closer the defect is to the toe, the shorter the fatigue life of the joint. For unfusion defects at the toe of the weld, the longer the length, the shorter the fatigue life. When the length reaches5 mm, the fatigue life reaches the critical value.