Abstract: It is very important to study the evolution law of fracture development in loess under humidification-dehumidification, because there are various cracks in the loess. Humidification-dehumidification lead to the development of internal cracks in undisturbed loess. The loess in Yan'an area is used as the research object. The soil microstructure change map of humidification-dehumidification and the soil sample CT number ME and SD values of soil samples with humidifier-dehumidification effect curve are obtained by CT scanning technology. The results show that: as soil moisture increases, the internal damage rate of soil is increased slowly, when the water content over the plastic limit of the soil sample. Soil internal damage rate increases slowly, when the moisture content exceeds a certain limit. The rate will fall. CT numbers mean and variance with moisture content changes are highly anti-"s" curve. With the dry-wet recycle number increases, the microcracks in soil samples, develop gradually and show elongation widened irregular bifurcation. After 4 dry-wet recycles, the ground fissures have been basically formed. The soil sample fracture and the CT number have little change. The more wet-dry recycle, the more damage degree of internal soil samples and development of internal joints and fissures in soil sample.