Based on the laser additive manufacturing technology, the thin-walled structural parts are formed. The method of negative defocusing of powder is adopted to solve the both-end collapse problem in the forming process of thin-walled structural parts. The microstructure and mechanical properties of thin-walled structural parts are analyzed. The results show that the ideal morphology of single track cladding layer is obtained at laser power of 1400 W, scanning speed of 0.6 m·min-1 and feeding speed of 9.5 g·min-1. The surface of the thin-walled structural part is free of powder and has no oxidation color when the lifting capacity of single layer is 0.57 mm. The height of cladding layer and the using efficiency of powder decrease with the increase of positive and negative powder defocusing. The microstructure of the thin-walled structural part is mainly dendrites growing epitaxially and the dendrites near the substrate are relatively coarse, however at the top is equiaxed grain structure. The hardness of the thin-walled structural part is higher than that of the substrate, and the hardness near the substrate is relatively smaller.