Abstract: In the process of atomic vapor laser isotope separation (AVLIS), the density and divergence of the evaporative atomic beam have a great influence on the selective excitation and the collection sub-processes. Based on the direct simulation Monte Carlo (DSMC) method, the simulation of the ytterbium metal evaporation process was carried out. Two typical structures of collimators, microchannel, and multi-layer slit plate, were considered under different conditions with low evaporation temperature (700~820 K). The results show that, in the microchannel, the divergence angle of the beam and the transmittance of the channel could be decreased by increasing the length-diameter ratio and the ratio of diffuse reflection or decreasing the inlet density. Besides, the minimum average divergence angle is 22.5°. In the multi-layer slit plate structure, the divergence angle of the beam and the transmittance of the channel could be decreased by increasing the distance between different slits, the slit width, and the ratio of absorption or decreasing the inlet density. The structure and wall interaction parameters of the last layer all have a significant effect on the parameters of the outlet beam. Finally, the parameter optimization methods of the collimator in the ytterbium evaporation device were summarized. The results could be used as a reference for the design of the evaporation device of metal with a high saturated vapor pressure at low evaporation temperature.