Abstract: In this paper, ultrasonic phased array technology was used to monitor the fatigue failure process of weathering steel butt joints. Based on the signal characteristics of ultrasonic probe, the fan-shaped scanning reflection process was studied, and a real-time scanning scheme was established for real-time monitoring of 10 mm thick weather resistant steel butt joints. The results show that when the fatigue life was 5 × 10⁴ cycles, multiple cracks initiated at the weld toe of the butt joint and propagated along the plate thickness were detected by phased array. The cracks begin to expand rapidly when the fatigue life exceeded 3.5 × 10⁵ cycles. Compared with the fatigue test fracture surface, it was found that the crack size based on the phased array detection was basically consistent with the test results, which validated the accuracy of the phased array crack dynamic detection. According to the relationship between the crack depth a, crack length c and cycle number N, the dynamic evolution of cracks was determined, the law of surface crack growth and evolution of weathering steel butt joint of medium and thick plate was built up.