Short staple fibers are processed in forms of bundle, and the thickness variation of the fiber bundles plays an important role in determining the bundle quality. Since the staples in a bundle generally have a length distribution, the processed bundle thickness is affected by the fiber length distribution, which is strongly dependent on the flow dynamics during the drawing process of bundles. This study examined the effectiveness of a dynamic model for describing the roller drawing process, while considering the power form of the beard diagram for the fiber length distribution of the bundle. Two factors, the drawing ratio and fiber length distribution, were considered, and the draft ratio was set in two levels: a low drawing ratio level and a high drawing ratio level. Slivers with different beard diagrams were treated under the roller drawing operation and the output sliver thickness was measured. In addition, the output thickness was simulated based on the theoretical model. The simulation results were compared with the experimental results. The theoretical model describing the bundle flow matched the real roller drawing operation quite well, which was confirmed by the agreement of the simulation and experiments results. A low drawing ratio of the output bundle is advantageous from a quality point of view. In addition, the bundle with a fiber length distribution resulted in better linear density regularity of the output bundle than the bundle with a uniform fiber length. However, the irregularity difference due to the fiber length distribution disappeared at drawing ratios above a critical value, between 20 and 30.