There are all kinds of fracture in the loess,and the development and evolution of the internal fracture in the loess under the action of humidification and dehumidification,which causes the occurrence of loess diseases.Sampling in Yan-an of Northern Shaanxi Province,the evolution law of soil samples under the condition of humidification process,dry-wet circle number and the amplitude of dry-wet circle was studied by CT scanning image technique.The results show that with the increase of soil moisture,soil internal damage speed of weak increase,when the moisture content is more than soil plastic limit,soil damage speed change larger,water content was more than a certain value,the speed is slow.The CT number of ME and SD with the water content change curve all showed anti "S" curve.And with the increase of the times of the dry-wet circle,soil micro fracture development elongated gradually widened,is not the rule of bifurcation,through; after 4 times of dry-wet circle,soil sample fracture is basically a mesh,the soil sample is broken,the CT number of ME and SD changes little.The greater the amplitude of dry-wet circle,the greater the degree of internal damage of soil samples,the more rapid development of internal fracture in soil samples.