Abstract: Formation mechanism of the type-II boundary of 1Cr18Ni9Ti/Q235 dissimilar steel joint with the filler metal of ER309 was studied. The results showed that the type-II boundary was formed by the migration of the solidified boundaries γ/γ from the position of the 5% ~ 6% Ni mass fraction in the unmixed zone to weld center. Under the condition that the partial fusion zone and the zone of Ni mass fraction less than 5% ~ 6%(N zone) in the unmixed zone provided the substrate of δ phase, or the partial fusion zone provided the substrate of γ phase and N zone provided the substrate of δ phase, the type-II boundary all could form near the fusion boundary of the carbon steel side. The type-II boundary could not form under the condition that the partial fusion zone provided the substrate of δ phase and N zone provided the substrate of γ phase, or the partial fusion zone and N zone provided the substrate of γ phase.