Abstract: The welding processes of transient flow and solidification behaviors were observed using high speed camera. The moving velocity of the interface between solids and liquids was calculated. The temperature variation of welding process was obtained by infrared thermal imager and the temperature gradient was calculated. The hot crack tendency of welded joint was investigated under three welding process of autogenous laser welding, laser-MAG hybrid welding guided by laser and laser-arc hybrid welding guided by arc. The results show laser-MAG hybrid welding guided by laser produced the lower moving velocity of solids and liquids, the lower temperature gradient, and smaller strain rate and sensitivity of hot crack. The microstructure and hardness were also tested in laser-MAG hybrid welding guided by laser.