There are many defects on the surface of continuous casting slab, such as corner cracks, longitudinal cracks etc. To analyze the cracks and determine their locations, two-dimensional heat transfer mathematical model and thermal elasto-plastic stress model of slab casting were established in this paper. The process of solidification, temperature field, and thermal stress distribution on slab surface were analyzed using finite element method. The effect of thermal stress on the cracks on the slab surface was discussed. The location of the cracks caused by thermal stress can be predicted. This is useful for finding approaches to overcome these defects.