The experimental research on soil water characteristics of expansive soil with different dry density is carried out by the pressure plate. The basic characteristics are analyzed on the soil water characteristic curve (SWCC). The effect of dry density is discussed on water retention capacity. The two basic eigenvalues are deduced according to the soil water characteristic curve. The results show that the effect is varied in the regime of SWCC. Dry density has little effect on the boundary effect stage when the matric suction is under the air-entry value. The water retention capacity is enhanced significantly on the transformation stage when matric suction is over the air-entry value. The air-entry value and resident water content are also high and increase as the dry density increases. The mathematic model is advanced to describe the SWCC of expansive soil. Then, the effect of dry density of expansive soil on the parameters of SWCC equation is analyzed. The relationship of dry density, matric suction and degree is described by a function expression.