Abstract: Super duplex stainless steel S32750 has excellent corrosion resistance and mechanical strength, and it is widely used in harsh corrosive environments.The welding heat input is generally recommended below 1.5 kJ/mm, but the reason is unknown.Welding tests of S32750 super duplex stainless steel were carried out under different welding heat input to observe the metallographic structure and precipitated phase in the welding heat affected zone(HAZ), the impact fracture was analyzed, and the relationship between the impact property and microstructure of welding HAZ was discussed.The results show that the impact property of S32750 welding HAZ decreases significantly when the welding heat input is bigger than 1.5kJ/mm, but no precipitate is found in the microstructure of the welding HAZ.The decrease of impact toughness is due to the coarsening of ferrite grains and the increase of HAZ width.