The effect of anhamonicity of layered transverse-optical phonons on the interayer pairing is investigated by a canonical transformation from a two-level-system model. The results show that the critical temperature Tc is zero only when the difference Ω of the two levels is smaller than a critical value Ωc, and Tc increases with the decrease of Ω. It is consistent with the importance of the axial motion of the ions in the insulating sheets as has been revealed by the recent experiments.