Using high-speed camera, experimentally studied the formation mechanism of hump defects of laser wire deposition 3D printing on 316L stainless steel for inclined wall unsupported structure. The results show that although the wire is deposited on the substrate in the manner of liquid bridge transition, it is still prone to hump defects when stacked in multiple layers; the size of the hump defects is about 4～9 mm,and the formation period is about 430 ms; under inclined conditions, the flow effect caused by the gravity of the liquid in the weld pool is enhanced, and the shear action of the liquid on the surface tension forms the hump.