A mathematical model was developed for the heat and mass transfer in vacuumizing process. Computational fluid dynamics technology was employed for simulation based on the unsteady model. The simulation results fitted experimental data well. After that, several parameters (pumping speed, free volume, and clapboard etc.) which would affect the flow field were analyzed. The higher the pumping speed is, the lower the pressure limitation is, the lower the minimum air temperature of vacuum chamber is and the more time spent in reaching minimum temperature. The free volume did not affect pressure limitation of the vacuum chamber. The greater the free volume is, the higher the minimum air temperature of vacuum chamber is and the less time spent in reaching minimum temperature. Referring to the pressure limitation and the average temperature of the flow field, the placement of clapboard can be equivalent to the reduction of free volume. 