Abstract: Coastal construction projects often subject soil to cyclic dry-wet conditions due to tidal fluctuations, which can lead to an unsaturated state that significantly affects the soil's load-bearing properties. This study experimentally investigates the interface shear characteristics between unsaturated soil and corroded steel, considering the varying roughness typical of coastal areas. Firstly, accelerated corrosion and laser scanning tests were conducted to determine the apparent roughness(R_a)of the interface on six steel plates with different levels of corrosion. Based on these results, a prediction model for interface roughness at varying degrees of corrosion was proposed. This model accounts for corrosion time, front-to-back quality variations due to corrosion, and corrosion solution resistance. Subsequently, 96 direct shear tests were performed on the interface between corroded steel and unsaturated clay, incorporating the Soil-Water Characteristic Curve(SWCC)of coastal unsaturated clay. These tests provided detailed insights into the effects of roughness, normal stress, and matric suction on the shear and dilatation characteristics of the unsaturated clay-corroded steel interface. The results showed that the soil consistently experienced shear compression, and the surface roughness of the steel plate initially increased and then decreased with corrosion time. The shear strength at the soil-interface was closely related to the interface roughness. At low roughness, the shear strength increased and then decreased with increasing moisture content, while at high roughness, the interface shear strength increased with increasing moisture content, but at a slower rate. Finally, a modified formula for the shear strength of the unsaturated soil-corroded steel interface was proposed, which considered the effects of interface roughness and matric suction. Experimental data validation confirmed the rationality and applicability of this formula.