•The droplet escape condition is established.•A three-dimensional numerical model is built to investigate spatter formation in laser welding of 5083 aluminum alloy.•The spatter formation mechanism and influence factors are analyzed. The droplet escape condition is established, a three-dimensional numerical model is built to investigate the behavior of the weld pool in fiber laser welding of 5083 aluminum alloy, the shear stress caused by the metallic vapor flow that has great influence on spatter formation is considered. A high speed photography system is used to capture the transient images of the weld pool, the spatter formation is discussed. The numerical and experimental results show that the spatters generate mainly around the keyhole. Two factors are responsible for the spatter formation: the surface tension of the molten metal around the keyhole is low. The recoil pressure caused by evaporation and the shear stress caused by metallic vapor flow accelerate the upward moving melt around the keyhole. Increasing the welding speed and placing the nozzle at the trailing position can decrease the upward momentum of the molten metal around the keyhole, and stabilize the keyhole, which are help to suppress the formation of spatter.