To provide theoretical guidelines for threshold value selections on performance characteristics of extension field, the flow characteristic in the vacuum-blowing cleaning system was simulated using the average velocity and pressure of the front inlet surface, and the average velocity of the outlet surface, as indices to evaluate the effect of the extension field’s structural parameters. It is found that the extension field parameters have implications for the simulation calculation, and that each parameter has a corresponding threshold. If the structural parameter is greater than the corresponding threshold, the calculation result is not affected, and the threshold values are analyzed by using computational fluid dynamics (CFD). The dimensions of the front, back, left, and right extension fields are recommended as follows: lf=lb=ll=lr=210 mm and θf =θb =θl =θr =55 degree. The flow field distribution characteristic does not have a distinct difference with or without the extension field corner. The extension field with a corner can be used if high accuracy is required. However, to reduce the amount of grid computation and shorten the calculation time, the corner extension field model is not recommended. Finally, the simulation results are verified experimentally and can be used to improve the calculation accuracy and reduce the required computational resources.