Abstract: Considering the desiccation cracks in the earthen sites,the crack initiation conditions,propagation characteristics and unstable propagation conditions of the desiccation cracks are discussed based on the section of Moon Bay Wall in Sanxingdui. The research results help reveal the cracking law of soil sites and have guiding significance for the engineering practice in earthen sites protection. The drying test is carried out for the soil of Moon Bay Wall. The sample water loss process is recorded by electronic balance. The cracking and expansion process of the crack is obtained with image analysis technology. The displacement field and strain field of the cracking process are obtained by digital image correlation technology. The results show that the surface defects of the sample reduce the tensile stress required for cracking. The smaller the distance between the defect and the boundary,the smaller the critical defect dimension of the crack initiation. Therefore,the crack is more likely to be generated near the boundary of the sample. According to the theory of fracture mechanics,considering the influence of water content on the soil properties,the crack unstable propagation need to meet the condition that the increase rate of stress intensity factor to water content should be greater than the fracture toughness. Theoretically,the calculation method for critical length of crack unstable propagation is derived,and the method is initially verified with measured data. As the water content decreases from 45.6%to 29.1%,the ratio between the critical length of the instability propagation to the width of the sample is reduced from 0.109 to 0.024. The lower the water content,the easier the crack enters the instability stage,and a small defect in the crack can trigger a rapid crack expansion. When protecting the surface of soil sites,the work should focus on repairing large-scale defects distributed in the boundary area of soil layers,should regularly monitor the surface of the soil sites. Once cracking is found,the cracks and their nearby defects should be timely repaired to prevent crack instability expansion.