Abstract: External constraint has a great influence on welding deformation and is one of the important factors affected on welding deformation. Compared with the traditional rigid constraint, dynamic constraint can reflect the effect of external constraint force in the welding process in real time. In the case of multi-layer and multi-pass cross welding of two welds, a multi-body coupling analysis model was established, and the dynamic constraint force effect caused by the deformation of the low-stiffness fixture due to the deformation of the welds was simulated by using the spring element and considering the feedback of the deformation to the constraint force. The deformation of welded parts were discussed from four factors: constraint stiffness, constraint distance, initial constraint force and constraint width. The results showed that the amount of deformation reduced at different degrees with the values of four restraint variable increasing. The influence trends of the constraint stiffness, the initial constraint force and the constraint width were similar, and the initial constraint force was the most important influence factor. The influence of the restraint distance was minimal and its trend was linear.