Abstract: In this paper,the characteristic equation of an annular absorption duct,has been derived,based on linear wave equation,and continuities of sound pressure and particle displacement across boundaries.Then the relationships between acoustic attenuation coeficient and some parameters were discussed.The acoustic attenuation coefficient will decrease with the increase of flow velocity,or with the decrease of absorption coefficient of boundary,or with the increase of section area of the duct.And an arched attenuation coefficient curve will be formed while the sound wave frequency changes from low to high.