The invention relates to medical chitosan fiber in the textile processing field and a preparing method thereof, the method for preparing spinning fluid comprises the following steps: arranging the chitosan powder in glacial acetic acid water solution and slowly dissolving by stirring under the condition of controlling temperature and time, wherein the dissolution temperature is 10-30DEG C, the dissolution time is 10-48h, and the stirring speed is 60-180r/min, wherein the chitosan powder occupies 3%-10%, dissolvent glacial acetic acid occupies 1%-5% and the other is water in the spinning raw materials, the chitosan content is more than 99.9% in chitosan fiber, and the chitosan fiber has higher molecular weight, sufficient amido, steady fiber structure, and good antibacterial property, brute force and hygiene security property.