To investigate the forming rules of laser additive manufacturing thin-walled parts on a nonhorizontal substrate and expand the extensive use of laser additive repair technology, the forming experiments of thin-walled parts were conducted on an inclined substrate from 0° to 150°, based on the inside-beam powder feeding technology. The forming rules of thin walls under different inclination angles were examined, and the forces of a molten pool under the nonhorizontal substrate were investigated. The experimental findings demonstrate that the total width and height of thin walls increase, and the length of the molten pool decreases with the increase of substrate inclination angle. The forming part’s tail collapse becomes severe with the increasing substrate inclination angle. A slope with a 21° maximum angle is obtained when the substrate angle is 150°. The study findings offer reference value for laser additive manufacturing and repair on the nonhorizontal substrate.