Abstract: In present paper, the form and mechanism of formation of out-of-plane distortion during bead-on-plate welding was studied by thermal clastic analysis with introduction of dummy element and experiment. Calculation result is fairly well agreed with experiment both in tendency and quantity order. Analysis shows that the out-of-plane distortion of weld fixed at one end is produced by angular distortion caused by transverse shrinkage and longitudinal bending caused by longitudinal contraction jointly, and that of longitudinal restrained weld is mainly produced by angular distortion caused by transverse shrinkage. For the weld fixed at both end, the deflection value of the whole plate is cut down. It is because that both transverse shrinkage and longitudinal contraction arc restrained to a certain extent.