Abstract: To explore the temper brittleness behavior of deposited metal for ultra-high strength marine steel, this study investigates the effects of different cooling schedules of post-weld heat treatment (PWHT) on the microstructure and mechanical properties of deposited metal of a 1 000 MPa-grade welding wire matched with deep-sea marine steel. The results show that the as-welded deposited metal presents a typical dendritic morphology with obvious element segregation. The inter-dendritic (ID) zone consists of martensite while the dendritic core (DC) zone consists of bainite. The deposited metal was heat-treated at 550 ℃-2 h and then treated by air cooling or furnace cooling separately. After PWHT, the ID zone transforms into tempered sorbite and the DC zone transforms into tempered troostite, which improves the microhardness uniformity of the deposited metal. The average ultimate tensile strength of the heat-treated deposited metal rises slightly with an increment from 30 MPa to 92 MPa. Its elongation remains stable between 15% and 17%, and the tensile fracture surface shows typical ductile fracture features. Nevertheless, the PWHT deteriorates the impact toughness of the deposited metal. At room temperature (25 ℃), the average impact energy of the as-welded deposited metal decreases from 125 J to 91 J (air cooling) and to 74 J (furnace cooling). At low temperature (-40 ℃), the impact energy reduces from 84 J to 65 J (air cooling) and to 33 J (furnace cooling). Slow cooling after PWHT causes coarse carbides to precipitate along grain boundaries and weakens the bonding strength of grain boundaries. Meanwhile, the fraction of high-angle grain boundaries (HAGBs) decreases. The barrier effect against crack propagation is weakened, which finally leads to an obvious deterioration of low-temperature impact toughness.