The hygroscopicity of the yellow dextrin/straw fiber composites at different relative humidity conditions was researched. The model of hygroscopicity and its variational rate was set up. The favorable water resisting property for the composites was obtained, on the two following groups conditions: the straw fibers of 100 g, the water of 40 g, the yellow dextrin of 20 g, the hot pressing temperature of 170℃, the hot pressing pressure of 4.2 MPa, the hot pressing time of 30 min; and the straw fibers of 100 g, the water of 40?g, the yellow dextrin of 10 g, the hot pressing temperature of 170℃, the hot pressing pressure of 9.8 MPa, the hot pressing time of 20 min respectively. The hot-pressing temperatures of two groups were both higher than in common. So the preparation process should focus attention on hot-pressing temperature factors. The research can provide reference for applying of the moisture absorption properties of the yellow dextrin/ straw fibers composites.