The assembled tube was made of beryllium tube and tungsten tube, and was produced by over fitting. The residual stresses in the assembled tube were very comprehensive after lasers braze fusion welding, not only including the welding residual stresses, but also the initial stresses from over fitting. Both of the dimension and strength were affected by the complex residual stress distribution in the assembled tube. MSC.MARC software was used to simulate the stress fields during lasers braze fusion welding of the assembled tube. The stress fields during lasers braze fusion welding and the residual stresses after welding have been obtained, and the effect of over fitting on welding residual stress of the assembled tube have also been studied. The residual stresses at outer surface of the assembled tube have been measured by the X-ray diffraction method. A comparison of the residual stresses by finite element method (FEM) to that by experiment indicates an identical stress change trend and thus validates the FEM model.