Abstract: This paper aims to explore the influence of physical and mechanical properties of expansive soil on swelling deformation. Based on principal component analysis(PCA),the paper studied the correlation between the swelling deformation of expansive soil and its physical and mechanical properties,and established the discrimination formula of swelling deformation with main physical indexes as parameters. It completed the above work through a large number of field geotechnical investigation and indoor test of an expansive soil project in Ankang,Shaanxi Province. The results show that the physical and mechanical indexes affecting the swelling deformation of expansive soil can be divided into the four types of factors. The first type of factors reflects the environment including water content,saturation,liquid index and water content ratio. The second type reflects soil structure characteristics including density,dry density and void ratio. The third reflects structural strength characteristics of soil mass including compressibility and internal friction angle. The fourth reflects soil composition including plasticity index and cohesion. Moisture content is the key index affecting the expansion deformation of expansive soil. The prediction model can provide a reference for the judgment of expansive deformation of expansive soil.