A series of wetting tests on a weakly expansive soil, taken from Huaian, Jiangsu province, was performed to investigate the swelling and compression deformations. The influences of initial water content and initial dry density were studied under the applied vertical pressures from 25 kPa to 800 kPa. It is shown that the swelling deformation due to wetting is mainly affected by the initial dry density, and little affected by the initial water content. At the same initial water content, the swelling deformation increases with the initial dry density increasing. At the same initial dry density, the swelling deformation decreases little with the initial water content increasing. According to the points of intersection of the compression lines on unsaturated state and the saturated state lines with different initial densities, a state line is obtained. The state line is almost not affected by the initial water content. And it can judge whether the expansive soil will swell or compress due to wetting under different void ratios and vertical pressures. Finally, based on the results of wetting tests, a simple method for predicting the swelling deformation of expansive soils due to wetting is proposed.