In order to explore the impact of culvert on the roadbed-culvert transition section in deep seasonally frozen region, a finite element model was established to simulate culvert’s temperature fields considering latent heat, and judged the reliability through comparing the simulation results with the existing monitoring data. Models of different parameters or different sizes were stimulated to investigate the influence on the maximum frozen depth. The results indicate that: 1) The measured temperatures and simulated temperatures agree well which demonstrated the accuracy of the numerical model. 2) In the design of culvert in seasonally frozen region, foundation convection under the culvert in cold season could not be ignored. Some active measures should be taken to protect the subsoil from the effect of convective heat transfer. 3) The culvert maximum frozen depth changing with the amount of water content is divided into three steps. At first, it decreased with increasing water content. And then, the reduction got slow, even the frozen depth increased slightly. At last, it decreased again; 4) The diameter of culvert greatly affected the maximum frozen depth, while the size of clear height had little effect on it. Diameter is priority for the requirements of strength, stability, functionality, etc. and the clear height could be considered at last.