In recent years, following rapid development of feed industry, a large number of synthetic feedstuff additives, e.g. antibiotics, somatotropin, hormone and preservative, have been broadly added in feedstuff. Application of such additives has greatly promoted development of animal husbandry. Following people's gradual understanding and unceasing improvement of food and environmental quality, they have found out the abuses hidden behind the huge economic benefits created by antibiotic feedstuff additives. For example, autogenous infection and superinfection caused by antibiotics, drug resistance of cells, decreased immunity of livestock and antibiotic residues in livestock products and environment has produced negative impact on livestock breeding, feed industry, and animal and human health. Therefore, many countries and regions have given orders of antibiotics prohibition. Through people’s unremitting efforts, some substitutes that are capable of effectively overcoming abuses of taking antibiotic additives and promoting growth have been developed successively, i.e. microbial feed additives, which are mainly produced from prebiotics and probiotics, and whose major function and mechanism is to strengthen animal immunity and stress capability, thus preventing and controlling diseases and promoting growth. 
    At present, people are trying to reduce pathogenic bacterium and improve animal health by increasing intestinal beneficial bacterium; and what is used most currently is viable preparations, which have evident effect on animals, especially young animals and sick animals, but also have the following insurmountable defects: (1) most viable preparations are anaerobic bacterium with difficult fermentation and unification of product quality standard; (2) most of them may be inactivated due to oxygen and high temperature during storage and processing; (3) they may be inactivated by gastric acid; (4) due to different living conditions of exogenous bacterium and intestinal bacterium, they have poor permanent planting in animal gut. 
    Aiming to defects of viable preparations, people have shifted their research on animal intestinal intrinsic bacterium in the hope of finding out some substances which can hardly be absorbed by livestock nor utilized by intestinal harmful bacterium, but can be absorbed by intestinal beneficial bacterium and promote their proliferation. Research proves that oligosaccharides has such function, and is called as “prebiotics” by people. The most typical representative is xylo-oligosaccharides, which is a non-digestive feedstuff additive with the capability of selectively promoting growth of colonic microorganisms and improve health of the host animal. It can hardly be digested and absorbed in animal stomach and small intestine, but can promote proliferation of beneficial flora in posterior intestine, especially bifidobacterium and lactobacillus, thus improving healthy growth of animals. 
    Oligosaccharide, also known as OS, is a type of short-chain or short-chain branch sugar. Due to its unique glycosyl group and glycosidic bond, it can hardly be digested, but utilized by intestinal beneficial bacterium, thus promoting proliferation of beneficial flora. It is named as “bio-probiotics” in the circle of nutrition. 
    Functional oligosaccharides mainly include xylo-oligosaccharides, isomalto-oligosaccharides, soybean oligosaccharides, fructo-oligosaccharides, and galacto-oligosaccharides, etc., among which, xylo-oligosaccharides is produced by corncob with enzyme conversion technology. Among oligosaccharides, compared with other oligosaccharides, xylo-oligosaccharides has its unique advantages, e.g. high selective proliferation of bifidobacterium, impossibility to be utilized by intestinal harmful bacterium such as clostridium perfringens, sound acid and thermal stability, as well as smaller dosage and better effect than other functional oligosaccharides. It is one of functional oligosaccharides with the most extensive development prospect, and enjoys high development and application value. 
I. Xylo-oligosaccharides Structure
Xylo-oligosaccharides, also named as XOS, is a type of functional polymer sugar composed of 2-7 xylose molecules bonded with β (1-4) glycosidic bonds. Its major functional ingredients include xylobiose, xylotriose and xylopentaose. 
II. Classification of Animal Intestinal Flora
According to secretion behavior of various bacterium in intestinal tract, bacterium may be classified into beneficial bacterium, harmful bacterium and facultative bacterium in between. Representative beneficial bacterium in animal intestinal tract includes bifidobacterium, fungus and lactobacillus; harmful bacterium mainly includes clostridium perfringens and staphylococcus, and facultative bacterium includes colibacter, Rickettsia-Lick bacterium, peptococcus and streptococcus. 
III. Unique Performance of Xylo-oligosaccharides
It selectively promotes bifidobacterium proliferation and can hardly be decomposed and utilized by gastric juice, pancreatic juice and small intestinal juice, with sound acid and thermal stability and small effective intake.