Abstract: Diffuser is an important component part for industrial ventilation device; the rational design of its structural parameters can reduce the fan outlet dynamic pressure loss, and improve the operating efficiency of the ventilation system. Taking 4-72 type centrifugal fan which is widely used in small mine as research object, its internal flow characteristics is studied by numerical simulation analysis method. On this basis, the detailed comparison analysis of the diffusion properties of two type straight-wall diffusers under different working conditions is made. Applicable conditions of two type straight-wall diffusers and the basic principles of the structural parameters selection are put forward.