In present paper, using Acoustic Emission (AE) as a ultrasonic technique, Welding crack has been investigated on the HG70 steel used by truck crane during three-point bending test was carried out. The study shows that the activity of AE increases greatly during the welding crack initiates and propagates, and the distributing range of AE characteristic parameters can be fixed. AE signals of welding crack initiation and expansion are typical sudden signals which have a different spectrum energy distribution range with that of unstable crack propagation.