Abstract: In this paper, the inner flow of perforated liner ducts is studied numerically by the computational fluid dynamics method. The fractional resistance characteristics of the ducts are analyzed. In the simulation,it is assumed that the air flow will not go into or out of the sound absorbing material through the pinholes. It is found that the combination of the realizable k--" model with an enhanced wall function can be used for the simulation of this kind of duct flows, and the pressure drop deviation between the numerical simulation and the experimental measurement is less than 10%. In the flow resistance calculation for perforated liner ducts in the engineering design, the concept of equivalent roughness is proposed, which can be calculated by the Colebrook equation and the Darcy--Weisbach Formula based on the pressure drop data obtained by numerical simulations.The in fluence of the size of the pinholes and the perforation rate of the perforated plates on the equivalent roughness is examined. It is found that the equivalent roughness is proportional to the square of the perforation rate, and is linear to the size of the pinholes.