Laser powder bed fusion (LPBF) is an advanced technology. However, the surface quality issue of the LPBF-ed parts deteriorates the mechanical properties. Laser beam defocusing (LBD) improve the surface quality with less time and economic costs. The effect mechanism of LBD on the surface quality needs to be revealed urgently. To reveal this mechanism, a LPBF numerical model is established and the laser beam power density distribution at different defocusing distances is studied. Based on the calculated results, the formation process of the surface defects is investigated. The surface defects and melt pool dynamic behaviors at different defocusing distances are discussed. Combined laser beam power density distribution, the effect mechanism of LBD on the surface quality is analyzed. The LBD improve the surface quality by (1) reducing the bulge height and depression depth, (2) reducing the spatters, (3) decreasing the partially melted powders.