Abstract: Aiming at the easy existence of bottom hump defect during laser deep-fusion welding,this paper designed a series of welding experiment of SUS 304 stainless steel plates with a single variable. Combining the welding phenomenon,detection signal and mechanism,the cause of the bottom hump defect was studied deeply to explore the relationship between the defect and the concentrated energy absorption region,and find the reasonable process parameters for restraining the defect. The result shows that the generation of the bottom hump is related to the concentrated energy absorption area in the keyhole. As the focus position descended,the concentrated energy absorption area showed a change rule of first moving down and then up,and the trend of the bottom hump defect weakened first and then increased. Therefore,a reasonable focus position can effectively restrain the generation of the bottom hump defect. In addition,the study of mutation trend of the bottom hump showed that there is a general rule that the concentrated energy absorption area moves down during the process of laser deep fusion welding,which provides a basis for the real-time monitoring of the laser welding state.