Abstract: To study the influence of ultrasonic peening treatment to welding cold crack, the tests were conducted on the specimens made of gray cast iron.Research focused on restraint stress and quenched structure of the three main reasons to induce the welding cold crack.Test method of cold crack was slit type cracking test. The test results show that by peening the whole weld, on the one hand, the ultrasonic peening treatment can reduce the residual stress at the most degree.That is the residual stress R₁ are -53 and -57 MPa, R₂ are -37 and -80 MPa on the two specimens weld.On the other hand, the ultrasonic peening treatment can accelerate to form the nodular graphite cast iron and eliminate white cast iron, and the weld is made of nodular graphite cast iron after ultrasonic peening treatment.The reasonable ultrasonic peening treatment can absolutely avoid welding cold crack by reduce the residual stress and quenched structure.