The invention discloses a method for preparing chitosan oligosaccharide by degrading chitose comprising steps removing extraneous constituents through fermentation culturing and separation purifying by utilizing shell dextranase bacillus TQK to obtain TQK enzyme liquor which enzyme activation is up to 718u/ml, putting TQK enzyme liquor into 10-20poncentration degrading liquor which uses chitose as bottom liquor, the ratio of TQK enzyme liquor to bottom object is 1:10, and the enzymolysis reaction condition is: mixing speed is 200-300 r/min, Ph is 4-5.5, temperature is 45-52 DEG C and time is 4-6 hours, then centrifugal separating, flocculating, fining, filtrating, ion exchanging, hyperfiltration stage treating, vacuum concentration disposing, sponging drying disposing to prepare chitosan oligosaccharide. The enzymolysis bubatrate concentration can be up to 15-20, enzymolysis time is only 4-6 hours, receiving rate is high, energy consumption is low, pollution is small, also the invention can prepare narrow molecular weight distribution chitosan oligosaccharide with average molecular weight D1500 in quack speed, low consumption and large mass.