Abstract: Bead welding experiment on Q235 mild steel plates with plasma-MIG hybrid arc was performed,and the space distribution of the arc,as well as the appearance and fine grain formation principle of the weld were discussed. The results show that the outer plasma arc is coupled with the inner MIG arc by the coactions on the filler metal tip during welding process. The temperature distributions in the arc space with the use of the hybrid arc are more uniform compared with that with the use of either one of the two arcs. The thermal cycle curve of the plates has a slightly high temperature span,indicating a fast cooling speed. With the increase of the plasma current in the hybrid arc, the wettability of the filler metal on the base metal and weld appearance are improved,and the weld width also increases. With the same heat input,the increase of the plasma current can improve the spontaneous nucleation of grains,thus the grains of the origin austenite and the weld are refined.