A novel method of testing cotton fiber length distribution was presented.The method include several parts as follows:mounting cotton fibers in the air pipe by air flow,grabbing fiber image through area CCD scanning them,and at the same time it effectively solves the problems of making cotton sample,forming optical image,processing image quickly,etc.The optical system presented in this paper,which makes use of the cotton properties and penetrating light,improves the image resolution that is usually constrained by the optic diffraction and makes it possible for the high testing accuracy and the less time consumption to be accomplished.The technique of mounting fiber and the optical system is original.