Laser – CMT (cold metal transfer) hybrid welding can increase penetration depth, enhance efficiency, and improve weld formation, yet the laser power is crucial for weld formation and process stability. In this study, high-speed imaging and electrical signal method were employed to investigate the weld formation and process stability during laser–CMT hybrid welding of a 6 mm medium-thick aerospace ultra-high-strength steel plate. The results show that the addition of laser shortens the droplet short-circuiting process, prolongs the boost and waiting times, and the droplet diameter slightly increases with laser power, yet the transition frequency remains essentially unchanged at 97 Hz. As laser power increases, the frequency of current instability increases, affecting the stability of the welding process. Analysis indicates that during the detachment and boost phases, the impact force of the droplet and the arc pressure impact and squeeze the keyhole, which is the main cause of keyhole instability. When the laser power is at 5000 W, the keyhole is fully penetrated, and the metal vapor force maintaining keyhole stability maintains a dynamic balance with the droplet impact force and arc pressure, resulting in the best formation with a porosity rate only 0.14%. However, when the power is increased or decreased, the formation deteriorates and porosity defects increase.