Abstract: Castanea mollissima seeds were used as materials to divide them for dehydration gradient treatment in this experiment. The germination rate and normal seedling rate of C. mollissima seeds under different water loss treatments were determined. The microstructure of embryonal axis and the changes of polysaccharides and proteins in cells were observed by paraffin section and plant tissue staining. The ultrastructural changes of hypocotyl were observed by electron scanning microscopy. The results showed that the germination rate and normal seedling rate of embryonal axis decreased rapidly when the water content dropped below 30%. The germ cells were larger and more loosely arranged, while the radicle cells were more densely arranged. The microstructure and ultrastructure of embryonal axis showed that significant cytoplasmic wall separation happen in the embryonal axis with 30% water content. The embryonal axis with 10% water content were found severely contracted and a large number of cells were degraded. The polysaccharides and proteins in cells aggregate first and then decompose with the decrease of water content. Castanea mollissima seeds are highly sensitive to dehydration, as cells lose water, they hydrolyzed polysaccharides and proteins to fight dehydration. The decrease of water content to 10% would lead to severe cytoplasmic collapse and even cell degradation, this may be one of the reasons for the death of Castanea mollissima seeds due to dehydration.