Abstract: In this paper, according to basic fluid dynamics theory and interacting shear flows (ISF) theory, following conclusion can be proved. There exists the best computational mesh in simulation of free shear flows. The best mesh design are orthogonal grid which grid line are parallel to the direction of ISF viscous shear thin layer streamline, in addition, the grid must be refined in the thin layer along normal. As for no optimal mesh, it will be difficult to capture the physics viscous effect in ISF viscous thin shear layer as the result of much more refined grid and viscous effect will be numerical. The conclusion were validated by simulation of an uncompressible free shear layer laminar flow.