Agricultural grade chitosan oligosaccharide (COS), is an excellent pure natural ecological agent. It resists to plant diseases by inducing plant resistance, so there is no drug resistance. Hailongyuan’s chitosan oligosaccharide is decomposed and refined by the modern biological enzymatic hydrolysis technology, can be used in plants widely, nontoxicity, no side effects, harmless to human and livestock, can be biodegraded, can not harden the soil, and can be slowly degraded as a small molecule carbohydrate, which has no pollution to the environment. COS can be used as crop immune inducer, plant vaccine and biostimulant.