Abstract: An approximately calculating method for an absorption coefficient of sound absorber with cavity was studied. The energy conservation equation in viscous-elastic material, relations expressions between effective densities, elasticity modules of the every layers and the relevant quantities of the compounds of that layer were deduced. By using a model of sound propagation in absorb medium layers the energy absorption coefficient were obtained. The spectra of absorption coefficients of wedge-shaped sound-absorber were calculated using the relative expressions and in references 1Acta Acustica, 1965; 2(4):192-197 and 2Journal of Tongji University, 1979(1):96-104. The calculated results shows that the three methods are effective for wedge-shaped sound-absorber; but for the wedge-shaped sound-absorber with cavities, the calculated results by the method put forward is closest to the experimental results among the three methods. As an example, the absorption coefficients of the plat sound-absorber with cavities were calculated. The simulated results can express the affection on absorption performance when the configuration of a cavity is changed, and are proved by experiments. An effective method which calculates absorption coefficients of sound-absorber with cavities is given.