In order to know the regulation of crack evolution,dry wet cycling test taking hefei expansive soil as the research object is carried out to study the expansive soil crack evolution under rainfall evaporation. The image recognition program is used to quantitatively analyze the crack rate of the area and observe the shape and depth of the lateral fissure. The results show that in the case of single dehumidification,the area crack rate,fracture fractal dimension and other morphological parameters and fissure depths gradually increase with dehumidification time,and the soil surface morphological parameters have the limit value. The wet cycle is characterized by repeated cracking and healing of fissures,which tend to loosen the soil structure,and the crack rate and fissure fractal dimension tend to be stable. The lateral slope instability is caused by lateral fracture development through lateral profile observation.