Abstract: In-plane plateau stress is an important index to evaluate the in-plane yield strength of honeycomb paperboards, which is sensitive to relative humidity. The effects of thicknesses, core layers, and liners on the in-plane plateau stress of honeycomb paperboard were investigated by experiments. The in-plane bearing capacity is undertaken by both liners and core layers of honeycomb paperboards. The in-plane plateau stress model of honeycomb paperboard considering the effect of relative humidity was proposed based on the longitudinal yield strength of honeycomb paper under different humidity conditions, which was compared with the experimental data. The results show that the thickness and liner materials of honeycomb paperboard have great influence on the in-plane plateau stress, on which the core layers has little influence; The model can effectively reflects the influence of relative humidity conditions on the plateau stress of honeycomb paperboards.