Abstract: Abstract: In order to determine reasonable measures to protect the expansive soil slope and explain the mechanism of crack development in expansive soils, it is very important and necessary to study the evolution of expansive soil under the action of climate in the field. In this thesis, based on the project of leading water from Yangtze to Huai River, an observation was conducted on the surficial and vertical crack development in the expansive soil slope exposed by excavation under climatic conditions so as to quantitatively examine the factors influencing crack development and evolution of expansive soil slope under climatic conditions. Exposure time, slope angle and degree of expansiveness of the soil are all considered in this study. The results show that the intensity of surface crack development of the expansive soil slope which is defined by the ratio of crack area to the total area is mainly controlled by the water content. The intensity of surface crack and moisture content of the soil are negatively correlated in the first drying cycle. The intensity of surface cracking increased with decreasing water content, and the gradient is related to the degree of expansiveness of soil. The higher the degree of expansiveness of the surface soil, the higher the gradient of the plot. The crack intensity factor increased over time, but the quantity of crack developed stabilized around a certain value. The time of stabilization of crack development is positively correlated with the degree of expansiveness of the soil. The non-uniform distribution of water content is the reason to crack development. The higher the degree of expansiveness of the soil, the smaller the corresponding fragment size. The process of crack growth was shortly affected by the rainfall. The maximum crack depth reached 150 cm when it was exposed for 360 days. The state of crack initiation is determined by the climate in the field. The rate of crack development of the expansive soil along the depth direction is positively correlated with atmospheric temperature and negatively correlated with atmospheric humidity, respectively. The process of crack growth in the vertical direction was divided into 3 stages, including Period I of slow development, rapid development period and Period II of slow development. Crack growth in the vertical direction in the rapid development period is approximately 96 cm, which is about 64% of the total crack depth, and the speed is 4-5 times that in the slow development period. The steepness of the slope has influence on development of the crack depth, which mainly occurs in the early stage of crack initiation. V-shaped cracks were developed along the depth of the slope, which is controlled by overburden stress and matrical suction. Under the same conditions of evaporation, deeper cracks developed in soils with higher expansive index. The cracks densely developed and intersected under the slope surface to a depth of 40 cm, but are mainly vertical and parallel when the depth exceeded 40 cm under the slope surface.