The plasma produced during deep penetration laser welding decreases the efficiency of laser energy. The difference of thermal movement speed between electrons and ions results in a voltage between the nozzle and the workpiece, and an electric current occurs if the nozzle and the workpiece are short circuited. The current increases with the increase of external voltage. The charged particles can be driven away by electric and magnetic fields if the magnetic field is applied in the direction transversal to the laser beam. In the experiment the nozzle is elevated during welding so that the welding mode changes from deep penetration welding to heat conduction welding. The increment of bead length by applying electric and magnetic fields indicates the improvement of energy efficiency if plasma above the melt pool is driven away. With stronger magnetic field, longer bead length is produced. The length of deep penetration weld reaches largest value at 25 V external voltage and decreases when higher or lower external voltage is adopted.