Laser-arc hybrid welding is carried out in the groove, when the laser-arc distance is 2 mm, the arc and laser-plasma are strongly coupled. From the beginning of the welding process to the end of the welding process, the arc and laser-plasma are inextricably linked. The plasma brightness is high, the spatter is increasing, and the weld pool is violently fluctuating; the results show that weld penetration is high but weld surface forming is poor. The laser and arc plasma are periodically coupled when the laser-arc distance is 4 mm. With the increase of the filling height at the bottom of the groove, the laser and the arc are not in the same horizontal plane, the distance between the laser and arc increases, and the arc and laser-plasma are separated; When the liquid at the bottom accumulates to a certain height, it flows to the laser action area due to gravity and surface tension; the liquid level tends to be horizontal, and the laser-arc distance decreases; the laser and the arc begin to couple, and the molten pool is joined together; the weld surface is well-formed, and the weld depth is slightly reduced. When the laser-arc distance exceeds 6 mm, there is no coupling between the laser and the arc in the groove. During laser-arc hybrid welding in the groove, the liquid level of the whole weld pool is inclined due to the accumulation of liquid in the weld pool. The range of the best coupling distance of laser and arc is narrow (~4 mm), and the weld penetration is high and the surface forming is good.