Abstract: The influence of constraint on welding deformation of Aluminum-Li alloy 5A90 T joint welded by dual-beam laser was studied. The constraint studied in this paper included two categories, one was applied to stiffener, and the other was applied to base plate. In the case of stiffener, the constraint was applied to the top and lateral face. The stiffener was fixed on the base plate by tack welding before the continuous welding began. Two situations were compared, one is that the constraint applied on the top and the lateral face were unloaded after tack welding, and the other was not. In the case of base plate, constraint was applied by pressing block, which was pressed on the base plate from two sides of the stiffener. The influence of the distance between the stiffener and the pressing block on welding deformation was studied. The computational result was validated by experiments. The deformation of stiffener during welding and after welding when constraint was applied on it was smaller than that condition that constraint was not applied. The longitudinal constraint applied to the base plate influenced the amount and the direction of bending deformation, but had little influence on the angle deformation of the T-joint. The constraint applied to the base plate in y direction during welding contributed to the reducation of angle deformation. The computational results of deformation direction and amount of bending deformation were consistent with experimental results.