Abstract: Cracks appeared on the surface of a Q345R steel heat recovery boiler steam drum butt joint. The causes of cracking of drum butt joint were analyzed by means of magnetic particle and ultrasonic testing, metallographic examination, hardness testing . The results show that the cracks were mainly concentrated in the weld and its adjacent heat affected zone, where the hardness was significantly higher than that of the base metal. The butt weld adopted asymmetric X-groove design, and input line energy for the weld was too high, resulting in the formation of network ferrite and coarse widmanstatten structure, which significantly increased the brittleness and stress concentration of the weld. Under the combined action of high temperature and alternating thermal stress, the crack propagated along the brittle phase, which eventually led to the cracking of the butt joint.