Several groups of direct shear tests of Nanning expansive soil samples were carried out by improved direct shear apparatus. The results of the characteristics of the ultimate shear stress and residual shear stress at the interface of expansive soil-structure are presented as follows: linear relation can approximately reflect changes between the both shear stress and the three factors: vertical load, water content and dry density, just different degrees from each other; increasing the vertical load from 25 kPa to 100 kPa (up by 300%) can cause the average increase of ultimate shear stress from 58% (for samples with 1.61 g/cm3) to 80% (for samples with 1.76 g/cm3), and an close average increase of 180% for the residual shear stress; increasing the water content from 14.1% to 20.8% (up by 47.5%) can cause the average decrease of the ultimate shear stress from 40% (for samples with 25 kPa) to 80% (for samples with 100 kPa), and the average decrease from 25% (for samples with 25 kPa) to 30% (for samples with 100 kPa)for the residual shear stress; increasing the dry density from 1.61 g/cm3 to 1.76 g/cm3 (up by 9.3%) can cause the average increase of ultimate shear stress from 92% (for samples with 25 kPa) to 138% (for samples with 100 kPa), and an average increase of 4% for the residual shear stress. Sensitive analysis was further made to explain reasons causing the differences of the both shear stress induced by the three factors.