Standard bar of ventilation is the equipment extensively used in the tobacco industry to calibrate the variety of testing machines. The bar?s performance that is usually affected by ambient conditions is numerically studied in this paper. Firstly, the geometry of standard bar is obtained by an optical microscopic 3-D measurement. Then, Solidworks is used to build the 3-D model of standard bar and Gambit is used to mesh the model. Finally, the performance of standard bar under variant ambient conditions is analyzed by Fluent. Compared with the experimental data, the numerical results are found to be quite accurate, and it is found that the ventilation rate increases linearly as temperature rise and decreases non-linearly with the growth of pressure.