Abstract: Due to particularity of heat source, laser and arc often interferes with each other during laser-arc hybrid welding, resulting in spatter and root humping. 590 MPa grade marine high-strength steel was taken as research object, and influence of arc length on spatter and root humping during laser-arc hybrid welding was studied. In order to deeply study generation mechanism of spatter and root humping during laser-arc hybrid welding, high-speed camera was used to observe droplet transfer behavior and root weld pool. The results showed that shortening arc length could reduce interference between laser and arc, improve stability of hybrid welding process and reduce tendency of spatter. The root humping was caused by poor penetration of keyhole and discontinuous flow of bottom weld pool. Formation of root humping was inhibited by shortening arc length of laser-arc hybrid welding, it was due to the fact that shortening arc length could reduce laser absorption by plasma, improve laser energy utilization, and increase penetration and stability of keyhole.Highlights: Effect of arc length on spatter and root humping in laser-arc hybrid welding was studied, method of high speed camera was used to observe flow stages of bottom weld pool, and formation reason for root humping in laser-arc hybrid welding was further clarified.