By the high resolution synchrotron radiation X-ray three-dimensional micro-tomography technology, the porosity in laser-arc hybrid welds is characterized, which is taken as the initial void volume fraction in the GTN mesoscopic damage model. The finite element models of mesoscopic damage mechanics for hybrid welded joints with and without reinforcement are developed, and the principal stress and the void volume fraction distribution of tensile joints are obtained. Via the metallographic structure analysis of tensile fractures, it is shown that the geometrical and material discontinuity is the important reason for joint failure.