Abstract: In order to investigate the effect of high temperature ultrasonic peening on fatigue performance of Q345 steel welded joints, the weld toes were treated by self-developed ultrasonic peening device at room temperature(20℃), 250℃, 350℃ and 450℃, respectively. The results indicated that when the stress ratio R was equal to 0.1, both the conventional and hightemperature ultrasonic peening treatments greatly enhanced the fatigue performance of Q345 steel butt joints. Moreover, compared with the as-welded joints, the degree of improvement of fatigue performance of Q345 steel welded joint treated at 250℃was the highest, up to 99.41%. In addition, the degree of improvement of fatigue performance increased first and then decreased with the increasing peening temperature under the influence of residual stress and stress concentration.