Abstract: A new rotated wire metal active gas welding process was put forward to improve molten metal flow and to refine weld microstructure. welding current, arc voltages and weld pool behaviors ware collected to analyze the effect of rotation velocity change on the joint microstructure when build-up welding on mild steel. The results showed that the rotation of the welding wire could add transition frequency of short circuiting transfer and improve the stability of the welding process. The calculation of the droplet transfer approximate entropy also proved the stability of the welding process was improved. The flow of molten pool was added by the additional motion of the welding wire and this can be seen from the tungsten particle tracer method results. The width of the bead firstly increased and then decreased. The joint microstructure could be refined obviously and the epitaxial growth of weld metal was restrained.