To better understand the hygroelastic property of wood cell wall, the transition of materials in the area between the S1 and S2 layers is considered in the modeling of cell wall hygroelastic properties. The concentric cylinder model is modified and employed for its compatibility with arbitrary additional layers in different scales. In order to explain the results of the cell wall model, the polymer contribution to the cell wall hygroexpansion is investigated first. Although the amorphous cellulose slightly affects the cell wall hygroexpansion, its influence on the softening of cell wall moduli is not significant. The contributions of hemicellulose and lignin to the hygroexpansion vary as the microfibril angle in the S2 layer changes. The results of the polymer analysis help to explain the effect of the interlayer. Compared to the model without the S1-S2 interlayer, the cell wall swells more in the transverse direction if the transition in the inner S1 layer (S1-part) is considered. For the case of transition in the outer S2 layer (S2-part), the effect reverses. Whilst, in the longitudinal direction, the S1-part amplifies the shrinkage and the S2-part suppresses it. The interlayer affects the cell wall moduli under moisture condition in the same way as under the dry condition. The modeled softening effects can be managed to approach the measurements by adjusting the thickness of the interlayer. We believe that paying more attention to the materials transition between cell wall layers can certainly help us to better understand the cell wall behaviors.