The invention provides a method for preparing a carbon microsphere by utilizing waste cotton fibre. The waste cotton fibre is taken as raw material, deionized water is taken as reaction medium, and washing, disinfecting, cutting into strips, fading, soaking, dispersing, heating, pressurizing and stirring in a reactor, product washing, drying and grinding are carried out, thus the cotton fibre carbon microsphere the surface of which contains an active group is obtained. The technological process is advanced, no acid-base catalyst is used, no environmental pollution is produced, the cotton fibre carbon microsphere has stable physical and chemical performances, multiple groups can be directly grafted on the surface of the carbon microsphere without modification, and functional fiber can be prepared when the carbon microsphere is blended with polyester fibre, polyphenylene sulfide fibre and polypropylene fibre, thus the method provided by the invention is perfect for preparing the active carbon microsphere by utilizing the waste cotton fibre, and an advanced method is provided for recycling the waste cotton fibre.