Abstract: This article investigates the low temperature toughness of welded joints and simulated HAZ of ultral low-carbon 9Ni steel which developed by quenching, intermediate quenching and tempering processed.The specimens were investigated by Charpy Vnotch impact test, optical microscope, TEM and SEM.The results show that the microstructure of lath martensite is coarsened and the low-temperature toughness is decreased in coarse grained HAZ of specimens single pass welding thermal cycled.The reversal austenite on boundaries of lath martensite appears in the dispersed distribution, and the grain will be fined, and low temperature toughness will be increased in HAZ of specimens multi-pass welding thermal cycled. It is desirable that low-heat input, multi-pass welding and low inter-pass temperature should be applied, which gives high low-temperature toughness of welded joints.