Abstract: The present work has been done to study the effects of surface-active sulfur powder on weld formation and weld microstructure during 10 kW-level high power fiber laser welding of thick plate. Observed results show that the surface-active sulfur powder increased the fluidity of molten metal and then the molten pool was elongated. When the defocused distance was negative, the active sulfur powder could increase the weld penetration and this influencing rule would not change under different welding speeds. When the defocused distance was positive, the active sulfur powder could increase the weld width. High power laser welding process could refine the grain size of weld and increase the ferrite proportion in weld. The added sulfur powder could refine the grain size and increase the ferrite proportion further.