Abstract: In nature, desiccation cracks are one of the major factors to cause the progressive failure of clay dams, soil slopes and clay liners. So it is urgent to develop the effective approach to study the mechanism of desiccation cracking. For this sake, a new DEM model based on the shrinkage mechanism of clay soils is developed. In the model, the radius, bond strength and stiffness of the grains are all varied with the water content, and the parameters are determined by matching the experimental data. Then, the initiation and propagation of desiccation cracks in a thin circular clay layer with rough boundaries are simulated using the software of PFC^3D. And the geometric parameters of surface cracks are quantified using the image analysis techniques to be compared with the experimental observations. The conclusions are drawn as follows: the simulated cracking mechanism includes the effects of tensile stress and flaws in the sample; the procedure of crack propagation obtained from DEM presents the character of three stages, and the evolution curves of crack parameters are similar to those from laboratory experiments; and the effect laws of the soil-base cohesion on the crack patterns are also in agreement with the results in literatures.