Abstract: The surface crack propagation dynamics of 12 mm thick weathering steel butt joints were monitored by using multi-element ultrasonic phased array to accurately describe the dynamic process of fatigue crack propagation in welded joints, and the geometric evolution between semi-elliptical crack depth, length relating with fatigue life was obtained. By using Abaqus, the finite element analysis model was established, with the geometric dimension of the crack as the input, the energy release rate G in seven directions of the crack tip was calculated, and then the range of stress intensity factor of the crack tip was calculated. The K value of the crack tip (θ = 90°) is compared with the analytical formula recommended in the BS7910, and the results were in good agreement with each other. Furthermore, the fatigue life of the specimen was calculated by using the simplified formula and the two-stage propagation formula, respectively. The results show that the two-stage crack growth model could accurately predict the fatigue life of butt joints and was in good agreement with the experimental results.