Abstract: To predict sound-absorbing performance of anechoic materials,the acoustic reflection problem of a viscoelastic layer backed with periodically rib-stiffened infinite double plates is studied.Firstly,the reason why structural theories of plates are not,applicable to viscoelastic plates is explained through comparing flexural waves with Lamb waves.Therefore,(visco-) elastic theory is adopted to deal with(visco-) elastic plates,and ribs are treated by structural theories of plates.The coupling between ribs and plates is solved by Hull's method.Solution of reflection field is obtained at last.The accuracy of present method is validated by comparing results of acoustic reflection from rib-stiffened double plates computed by structural theories of plates.The influence of a backing on acoustic reflection of the viscoelatic layer is analyzed by computing reflection coefficients.Performance of different viscoelastic materials are evaluated by defining the average reflection coefficients.The results show that,influence of a backing on acoustic reflection cannot be suppressed by viscoelastic materials in low frequencies.The resonance is introduced by coupling of the fluid layer and double plates.And ribs,which are coupled with double plates,mainly make acoustic reflection reduced.