In order to study the formation process of keyhole, in the fiber laser plate scanning welding of low carbon steel, experiment was carried out by modulating laser light-out time. Experimental results show that the formation process of keyhole is extremely rapid, and the whole formation time of a keyhole is in ms level. In low-speed welding, the formation process of keyhole includes three stages: rapid increase, slow increase, and roughly stabilized stage. During high-speed welding, the formation process of keyhole only includes rapid formation stage. Further experiments show that the formation time of keyhole is not exceeding the characteristic time of laser welding and the characteristic time of laser welding decreases with the increase of welding speed, which together explain why keyhole formation only includes the rapid formation stage in high-speed welding.