The properties of plain fiber assembly are discussed, and the fiber alignment in the assembly is investigated. Some theories in yarn irregularity research are applied to the study of the random and parallel fibers in fiber assembly and a stochastic method is used to generate the fiber assembly. For a given fiber length distribution, the combing process is simulated on the basis of the plain fiber assembly. If the main parameters in cotton combing are known, the simulation can be conducted, yielding the fiber length distributions in combed cotton slivers and in waste cotton that is combed down. With the variation of combing parameters, the influence of combing parameters on the combing results can be discussed from the simulation.