A method for preparing chitosan oligosaccharides through semi-wet method microwave treatment relates to a production method of chitosan oligosaccharides. The method comprises the following steps: using chitosan gelatine powder as raw material to perform hydration and microwave treatment, dissolving in water, neutralizing, and performing membrane separation, drying and the like to prepare chitosanoligosaccharides. The method does not consume organic solvent, thus the problem of recycling solvent does not exist and the pollution is low; and as the volume of the reaction system is from 1/3 to 1/5 of that of the solution method, the production efficiency is obivously increased. The method adopts microwave treatment to promote the polysaccharide degradation reaction, thus the heating effect is greatly increased. The radiation thermal effect of microwave and the non-thermal effect of microwave are utilized to prepare the product. Meanwhile, microwave is an efficient and clean energy, thus the speed of the chemical reaction can be greatly increased. The method also adopts the membrane separation mean, thus the refining efficiency is high and the product quality is controllable.