In this research, a 3-D thermal-elastic-plastic FEM is developed to simulate the out-of-plane deformation caused by one-sided automatic submerged arc welding. Efforts have been made to improve the reliability and accuracy of the FEM calculation. To prevent the phenomenon of transverse-shear-locking, which is observed as overestimation of the stiffness of a plate for a bending problem when solid elements are used, reduced integration method is employed and proven to be effective. A special type of spring-element is utilized to deal with the contact problem between the plate and the working table. Using the 3-D FEM, the effects of various factors, such as gravity, the method of supporting, magnetic constraint and force from the backing near the welding line have been investigated. The results from the numerical simulation can reasonably explain the phenomena observed from the real situation in shipyards. In the case of FCB automatic submerged arc welding which has the feature of large heat input, out-of-plane welding deformation is mainly affected by constraint condition. The gravity of the plate and the backing force have great influences on the shape of angular distortion. Magnetic constraint can reduce the welding deformation effectively.