Based on the two-dimensional heat transfer and solidification model, the differences in heat transfer boundary conditions in the casting direction and width direction of a slab are researched, and the results are verified by experiment. By using the method of "limited thickness slice unit", the numerical discretization of each slice unit in the computational domain space is carried out in real time, and the casting information of all the slice units in the slab is tracked in real time, which achieves the effect of a three-dimensional temperature field simulation model. Through the double process program, the three-dimensional temperature field on-line simulation model calculation cycle meets the on-line control requirements. The three-dimensional temperature field on-line simulation model is successfully applied in a domestic steel mill to the 4th slab caster. The secondary cooling dynamic water model and dynamic soft-reduction model based on the three-dimensional temperature field are used to modify the original model. After modification, the qualified rate of crack detection feedback from rolling is improved by 1.4%, and the qualified rate is basically stable at approximately 99.5%. The incidence of transverse cracks, longitudinal cracks, and corner cracks on the slab surface decreases by 53.96%, 56.3%, and 29.19%, respectively.