In this study, cellulose and xylan were in vitro fermented by pig fecal bacteria. Rapid fermentation (40 h) and extended fermentation (eight weeks) were performed. The properties and ultra-structure changes of post-fermented solid residues were studied. In the end effluent, acetic acid, propionic acid, and butyric acid were observed to be the principal short-chain fatty acids (SCFAs) produced by anaerobic fermentation. Xylan was more accessible to bacteria than cellulose, leading to higher SCFA and lactic acid production. In addition, the crystalline structure of cellulose changed, leading to 16.3% and 42.1% increases in crystallinity index for rapid and extended fermentation, respectively. Through this research, a systematic and advanced method to study the degradation chemistry of cellulose and xylan during fermentation was developed.