Selective laser melting (SLM) is a widely used high-precision additive manufacturing technology that can achieve arbitrarily complex structures. The powder size used by SLM is generally 15-53 μm, which is suitable for manufacturing parts with a forming accuracy within tens of microns. However, the reason why smaller or larger particle size powders are not suitable is not yet clear. The effect of particle size on SLM processability was studied by simulation and experimentation. Three powder particle sizes of AlSi10Mg were used to study the behavior of powder spreading and melting/solidification during SLM by discrete element and computational fluid dynamics methods, respectively. The macroscopic forming quality of the formed samples was tested. The results show that the powders with a particle size below 20 μm agglomerate vigorously to form many cavities, and the powders with a particle size above 53 μm tend to form few large cavities. The relative density of the powder bed with the medium particle size is 7.69% and 3.17% higher than those of the fine and large particle sizes, respectively. The melt channels of the fine and coarse particle sizes are irregular due to the uneven quality of powder laying when the powder bed is melted. However, after multilayer melting, defects in the melt channel of the fine particle size are partially alleviated. With the increase in particle size, the melt channel surface flatness decreases, the fine particle size powder samples have more porosity, and the coarse particle size powder has a few unfused defects. The processability of the medium particle size for SLM is the best among them. The relative density of the sample with the medium particle size reach 99.8%, which is 1.4% and 0.4% higher than those of samples with fine and coarse particle sizes, respectively.