The effect of applied laser and magnetic field on narrow gap arc welding of 10 mm thick SUS316L austenitic stainless steel was studied. The results show that the arc in narrow gap arc welding is difficult to be stable in the narrow gap, and the sharp Angle at the bottom of groove cannot be heated. Under the appropriate laser power, the arc in the narrow gap can burn stably. However, too large a laser parameter will cause the arc to move up and will not be able to heat the sharp Angle at the bottom of the groove. Under the action of constant magnetic field, the arc in the narrow gap swings to one side, beyond the narrow gap range, and the other side cannot fill the narrow gap. With the increase of magnetic field strength, the asymmetry becomes more obvious. Under the action of alternating magnetic field, the left and right skewness of arc appears alternately, and the shape is almost the same as that of constant magnetic field. Under the action of laser, the laser induced plasma causes the arc to gather and burn stably at the hole. When a magnetic field is added, the charged particles rotate, and their uneven distribution in the arc causes the arc to aggregate on the left or right side. Therefore, a change in the direction of the magnetic field causes the arc to swing alternately on the left and the right sides.