In order to obtain metal parts with high density and accuracy,the balling phenomenon in selective laser melting（SLM） process should be eliminated.In this paper,the evolution mechanism and weakening method of balling are theoretically analyzed,finding that the change of processing parameters,such as laser power,scan speed and powder layer thickness can effectively weaken the balling because there is sufficient melting amount of the substrate.When the substrate is melted to a certain degree,the fresh molten pool is divided into an upper pool formed by metal powders and a lower one formed by a part of substrate.The former always has a tendency of balling due to the liquid-gas interfacial tension,while the latter without balling can obstruct the balling course of the former.Thus,if there is enough metal liquid in the lower pool,the balling of the upper pool during the SLM can be completely eliminated.The above-mentioned statements have been validated by the SLM experiments at a laser power of 150W or 80W and various scan speed and powder layer thickness,with Cu-based alloy powders in an ave-rage diameter of 75μm as material.