The soil-water characteristics of expansive soil are influenced by cracks caused by drying-wetting cycles. Electrical conductivity tests were carried out to obtain the soil conductivity without cracks and electrical conductivity had a linear positive correlation with soil dry density and water content. A linear multivariate function was obtained to describe their relationships. A long-distance microscope system was used in observing the crack growth of the expansive soil dynamically under the conditions of drying-wetting cycles and the gray level entropy of image with the cracks was used to evaluate the crack shape. The soil water content and conductivity were obtained and the gray level entropy-water content-conductivity relationship of soil was established under the conditions of the drying-wetting cycles. The gray level entropy increased and conductivity decreased with the crack growth due to water loss. The impacts of water content on the soil conductivity decreased and impacts of cracks on the soil conductivity increased gradually. The impacts of cracks on the soil conductivity kept constant under drying to a certain degree, and the shape of cracks remained stable; the cracks were closed partially due to water absorption. The gray level entropy decreased and the soil conductivity increased. The gray level entropy and the soil conductivity decreased to some extent under the drying-wetting cycles. It was shown that the soil structure was changed irreversibly.