Abstract: Cracks are common in nature and affect a number of soil properties. They increase the permeability, reduce the soil strength and stability, and thus affect the growth of plant roots. This paper presented a model for crack formation which was introducing the correlation length to constructing the soil critical strain random field to optimize the cracking model. The model was based on a lattice of Hookean springs with finite strength and the critical strain. The concept of correlation length was introduced to constructs the critical strain random field, the correlation length was used for reflects the spatial continuity of the soil. The critical strain random fields were generated using program written by Fortran language. The model reproduced the dynamics characteristic of the development of crack network in nature, and some of the parameters of the model represented the physical properties of the soil. We processed the digital images to quantitatively analyze the geometric characteristics of cracks. The typical procedure of digital image processing included: the color image of the crack patterns was converted into a gray one, the gray image was further changed into a binary one with the threshold division method. After the process, the cracks and aggregates were simply distinguished in a black-white image, in which the black lines and the white areas represent the crack networks and the aggregates, respectively. We optimized the threshold to make the binary images as clear as possible, and processed the images through binarization, noise removal and edge extraction of crack for extraction of morphological properties. Finally, the influence on model simulation cracking of spatial correlation length, heterogeneity in random field was analyzed. Comparison of crack area, crack length, Euler number density by calculating experiment and simulation images, it could be drawn that crack morphology could be described by Minkowski area, length, Euler number density effectively. By calculating the average μ, standard deviation δ of cracks area, length, Euler number density, the images of average density value μ and μ±δ are basically the same in the change trend between experimental and simulated process. After calculation, the coefficients of determination R2 for the area, length and Euler number density are all larger than 0.5, and the root mean square error and deviation are small enough. The consistency index IA quite approach to 1, which indicating that the calibration results are satisfactory. The two models are compared, among them the determination coefficient R2 is between 0.889-0.968, the consistency index IA is between 0.964-0.993, which reflecting the dynamic cracking process of cracks well. The greater the correlation length and the smaller the variance, the lower the discretization degree of the random field, that the more uniform the soil cracking. On the contrary, there are mainly isolated cracks. And the mean value is too small, the whole strain of the random field is low, the simulated crack does not consistent with the development of the natural crack.