Abstract: Dissimilar steels with thick wall welded by ultra-narrow-gap welding have some extra advantages in increasing welding efficiency, reducing production costs, improving mechanical properties of joint. In the paper, microstructures and mechanical properties of welding joint of Q235/1Cr18Ni9Ti dissimilar steels with ultra-narrow-gap welding were studied. The results indicated that welding pool of backing weld solidified with the FA solidification mode, and solidification microstructures were made of fine equiaxed grains of austenites and a few dendritic ferrites. Welding pool of filling bead and cover pass near the transition region of fusion solidified with the AF solidification mode, solidification microstructures are made of cellular austenites and ferrites between cellular austenites, welding pool of filling bead and cover pass far away from the transition region of fusion can solidify with the FA solidification mode, and solidification microstructures were made of columnar dendrite of austenites and a few skeleton ferrites. Ultra-narrow-gap welding joint of dissimilar steel had excellent tensile strength and bending property, and the heat affected zone near the Q235 base metal can not soften. Impact toughness of the transition region of fusion and the heat affected zone near the Q235 base metal was better than that of weld zone.