Abstract: In high speed gas metal arc welding (GMAW) process, severe geometric defects such as humping bead and undercutting will occur if the welding speed exceeds the critical value. The previous experiments had shown that the backward flowing molten jet with high momentum in the weld pool is responsible for the formation of humping bead. In this study, an external tranverse magnetic field generator is developed so that the welding current in the weld pool interacts with the magnetic field to produce an additional EMF pointing toward the front part of weld pool. And the EMF can control the backward flowing molten jet in the weld pool and adjust fluid flow field to suppress the occurrence of the humping bead. Bead-on plate welding experiments were performed and the influences of the exerted magnetic flux density on the weld bead formation, the arc and the fluid flow behavior inside the weld pool were investigated. The mechanism of the suppressing humping bead by the external electromagnetic field was revealed. The results show that the external magnetic field can remarkably adjust the fluid flow field in weld pool and prevent the occurance of the weld defects such as humping bead and undercutting, so the quality of weld bead is remarkably improved, and the critical welding speed is notably increased.