Ultra-high power laser-MAG hybrid welding technology provides an effective method for joining thick plates with high-quality and efficiency. Heat source configuration of laser and arc was verified to play a vital role in conventional hybrid welding process. In this paper, ultra-high power laser-MAG hybrid welding was utilized to weld 20 mm thick Q235 steel for clarifying the effect of heat source configuration on welding process and joint formation. By high-speed photography, welding process was systematically recorded including hybrid plasma morphology, droplet orientation-transfer and molten pool flow. Subsequently, the joint formation was further analyzed via dynamic behaviors of liquid column and keyhole. Results exhibited that a stable arc characteristic and smooth molten pool flow was formed under HM (ultra-high power laser and MAG-leading) mode. Besides, the enlarged and stabilized keyhole would inhibit the height of liquid column and formation of spatters. Finally, a sound weld joint without “ant-colony-like defect” joint formation and 15.82 mm penetration was produced in HM mode. This work provides an effective method and reference for realizing thick plate single pass weldments in the practical industry. Display omitted