How to select this face as the inlet bc.
 

I use the following stl file for the snappyMesh. but i need to set the inlet bc as shown in the picture.
Could you please give me some advices about how to select this face.

The "easiest" way to do this, is to use an stl file with regions. These regions are maintained by snappyHexMesh during the meshing and you do not need to reselect them. However, most programs do not produce so called watertight stl files, which are needed for snappyHexMesh (in most cases).
You can read about watertight stl generation with regions in this thread: https://www.cfd-online.com/Forums/op...sh-salome.html