The molecular and physical structures of cellulosic rayon fiber during the stepwise heat treatment for the fabrication of activated carbon fiber (ACF) were researched by Fourier transform infrared spectroscopy, wide angle X-ray diffraction and elemental analysis. Dibasic ammonium phosphate (DAP) lowers the dehydration temperature of rayon evidently and acts also as a reactant besides as a catalyst. Group C=O is generated at 200 °C while group C=C appears at 250 °C. About 220–250 °C is a key temperature range where the chain, ring and crystal structures of rayon macromolecules are damaged completely to lose the basic characteristics of cellulose by dehydration. The drastic changes of weight loss, discoloration and shrinkage occur simultaneously. The bands of chemical groups decrease rapidly and go to disappear during carbonization. The activation at a lower temperature (750 °C) decreases the contents of oxygen-containing groups, but increases them at a higher temperature (950 °C). The microcrystalline structures are gradually formed near 800 °C and activation seems to be favor to the crystallization. A higher usage of the phosphate does not only affect the weight-losses of rayon fiber significantly, it also enhances the specific surface areas of ACF obviously.