Abstract: An embodiment of a sheath flow device may include: a reflecting plane, aspheric reflecting surface, and a conduit for passage of cells disposed in a central hollow. A focal point of the aspheric reflecting surface is positioned in a location where cells settle. A collimated beam generated by an external light source incident on a first reflective side of the reflecting plane, is reflected to reach a first reflective surface of the aspheric reflecting surface, and reflected before being focused on the focal point, after which the light proceeds to reach a second reflective surface of the aspheric reflecting surface to be reflected to reach a second reflective side of the reflecting plane to emerge after being reflected thereby. The first and second reflective sides and the first and second reflective surfaces may be centrosymmetric. The sheath flow device improves the alignment accuracy of beam commissioning during the application process.