Xylan is a kind of poly pentose，the main chain of which is composed of several pyran xylose base connected by β-1, 4-xylose glycoside bond. And the side chain connects many different sizes of short substituents. Generally speaking，the substitute of arabinose residue in xylan will happen to C-2 or C-3 of some xylose residue, so it's also called Araboxylan. The substitute of araboxylan debases the acting force of the main chain's chemical bond，thereby make it bear water-solubility and slumpability. The higher degree of arabinose base substitutes in Araboxylan，the better its water-solubility will have. According to solubility, xylan can be divided into two main sorts, i.e., water-solubility xylan（occupies about 25% ~ 30%）and infusibility xylan（occupies about 70% ~ 75%）