A fluid dynamic model was set up to describe the flow field of gas in a large sized pit type carburizing furnace when large sized gears were being carburized. The commercial software Fluent was adopted to carry out 3 dimensional computational fluid dynamics (3D-CFD) simulations of the gas flow field under different, actually four kinds of , furnace designs in this article. The flow fields of the carburizing gas around the part were analyzed. According to the simulations and analysis, it was shown that the number of fans on gear’s carburizing is not a primary factor, using a air inducting tub can improve the carburizing process significantly and proper loading tray design can also be positive. The results indicate that the simulation provides a reference to the furnace’s design optimization.