Abstract: The interaction between clay particles during the process of desiccation cracking is highly complex, making it challenging to conduct quantitative researches on the formation mechanism of cracks at the particle level. The capillarity and adsorption in clay is distinguished based on the suction stress characteristic curve (SSCC) in unsaturated soils. A contact model of discrete element method (DEM) that accounts for the change of attraction between clay particles with water content is then established, and the numerical simulation of desiccation cracking in clay is carried out. The simulated results are compared with those of laboratory tests, and the findings indicate that the crack morphology, crack development history and strain of the soil sample obtained by the DEM simulation are in good agreement with the laboratory results, verifying the reliability of the DEM model. Further analysis of the simulated results reveals that: (1) The capillarity and adsorption both play a role in the desiccation cracking process of clay. With the decrease of water content, the effects of adsorption gradually exceed those of capillarity. At the dominant stage of adsorption, the average displacement of simulated soil particles along the crack distribution direction accounts for 73% of its final value. (2) The total contact number between soil particles initially decreases and then increases as water content decreases. (3) The physical contact force between soil particles will increase rapidly at the dominant stage of capillarity/adsorption, resulting in stress concentration, and the contact between soil particles will be centralized and cracks will be formed. The proposed DEM contact model is of significant physical implications and can offer valuable insights into the underlying mechanisms of desiccation cracking in clay at the particle level.