The effects of Reynolds number，liquid head，orifice diameter and plate thickness on the flow characteristics of vertical sharp-edged orifice discharge were investigated experimentally.According to the distinct profiles of orifice discharge coefficient，it was found that the orifice discharge followed two mechanisms of “small orifice” and “large orifice”，and the orifice coefficients were analyzed regionally.The dissipation of mechanical energy in the orifice flow process with respect to “small orifice” and “large orifice” and the energy loss in different operating regimes were discussed on the basis of the boundary layer theory.