NOVELTY - Preparing chitosan oligosaccharide involves mixing golden cypress, clove, wormwood and bulbul in ratio of 2:2:1:1 under stirring condition at 60-70 degrees C at 450-550 revolutions per minute (rpm) to obtain mixture. The mixture is subjected to ultrasonic treatment at 30-40kilohertz for 1-2 hours. The obtained reaction product is heated and filtered to obtain filtrate. The filtrate is added with 75% alcohol at room temperature and subjected to ultrasonic treatment at 50-60kilohertz with stirring at 100-200rpm for 20-30 minutes. The obtained product is placed at 0-4 degrees C for 4-8 hours and then filtered.    USE - Method for preparing chitosan oligosaccharide (claimed).    ADVANTAGE - The method enables to prepare high yield of chitosan oligosaccharide, which has enhanced disease resistance of crops, and increases yield and quality of product, and is eco-friendly.    DETAILED DESCRIPTION - Preparing chitosan oligosaccharide involves mixing golden cypress, clove, wormwood and bulbul in ratio of 2:2:1:1 under stirring condition at 60-70 degrees C at 450-550 revolutions per minute (rpm) to obtain mixture. The mixture is subjected to ultrasonic treatment at 30-40kilohertz for 1-2 hours. The obtained reaction product is heated and filtered to obtain filtrate. The filtrate is added with 75% alcohol at room temperature and subjected to ultrasonic treatment at 50-60kilohertz with stirring at 100-200rpm for 20-30 minutes. The obtained product is placed at 0-4 degrees C for 4-8 hours and then filtered to obtain filtrate residue. The filtrate residues are dried at 40-50 degrees C and then crushed to obtain medicinal extract after 3-6 hours. Bacillus subtilis (1-5 pts. wt.), Bacillus megaterium (0.2-2 pts. wt.) and yeast (0.5-3 pts. wt.) are cultured in culture medium to obtain multi-strain culture broth, which is added with chitosan (35-45 pts. wt.). The multi-strain bacteria is added to fermenter and fermented at 32-35 degrees C for 5-8 hours to obtain mixture having moisture content of 35-40%. The obtained medicinal extract (5-8 pts. wt.) is added to the fermenter with each time interval of 2-3 hours. Fermentation is carried out at 32-35 degrees C for 8-12 hours to obtain fermented product. The fermented product is filtered to obtain filtrate. The filtrate is placed at 65-75 degrees C at 450-550rpm. The filtrate is subjected to ultrasonic treatment at 50-60kilohertz and heated for 40-60 minutes. The obtained product is filtered to obtain filtrate. The filtrate is added with 75% alcohol and allowed to stand at 0-4 degrees C for 4-8 hours. The obtained product is filtered to obtain filtrate. The obtained filtrate is concentrated and dried to obtain chitosan oligosaccharide.