Abstract: Cracks are one of the most significant influences on physical and mechanical properties of expansive soils. Some expansive soils, taken from Nanyang Expressway, are compacted to large-size samples and then dehydrated gradually in a closed greenhouse with various ambient air humidities and temperatures for desiccation tests, and a camera is utilized to record the development of cracks on the surfaces of samples. The captured images are disposed and then analyzed quantitatively to obtain typical characteristic parameters of surface cracks, such as crack ratio, average width and total length, for further exploration of crack developing laws. The results show that the cracks observed from the large big-size samples are more similar to those on the site such as embankments and slopes compared to the ring-knife-made or thin-saturated-slurry samples. The higher the ambient air humidity, the slower the cracks develop in the early stage, and the longer the developing time lasts, while the larger the statistical characteristic parameters of surface cracks in the later stage. The higher the ambient air temperature, the faster the cracks develop in the early stage, and the faster the cracks retract and become stable, while the smaller the statistical characteristic parameters of surface cracks in the later stage. Low ambient air humidity or high ambient air temperature makes it easy for the surface of soil samples to generate more invisible micro-fissures, which is an important reason for the small statistical characteristic parameters of cracks compared with the theoretical analysis. High ambient air humidity or low ambient air temperature may be beneficial to the transfer of shrinkage stress to deeper soil layer, and promote the development of cracks with fewer branches, larger width and deeper depth.