Abstract: Using the basic principle of probabilistic density functions, three cotton fiber length distributions by number (including one input and the two outputs cotton fibers) are compared, deducing the changing rule of the fiber length distribution during combing. The conditions under which fiber length distributions at both input and output should be met is discussed. The characterization curve of combing effectiveness of the combing machine is obtained by computation using solving linear equations. The work has shown that the computation method based on fiber length distributions can effectively evaluate the working efficiency of the combing machine. Furthermore, a reasonable indicator is given so that the fiber breakage during combing can be characterized. The findings can be used in optimizing the combing process and estimating the fiber breakage during combing and as a reference for the design of combers.