In this paper, a three-dimensional mesoscopic model of the laser energy absorption behavior in the powder layer is established based on the ray tracing using COMSOL software. The effects of Ti6Al4V powder particle size, powder layer thickness, powder layer spatial distribution and particle size distribution on energy absorptivity in selective laser melting (SLM) are investigated. Simulation conclusions indicate that the absorptivity of closed-packing model is between 63.06% and 63.24% with particle size increasing from 5 μm to 60 μm; while the absorptivity of loose-random-stacked is between 69.37% and 70.46%, and the fluctuation of absorptivity along the scanning path sharply increases from 0.68% to 1.99% when the particle size of the powder increases from 20 μm to 25 μm. The absorptivity is insensitive to the changes in powder particle size; however, considering the stability of absorptivity during the SLM, the particle size of the powder should be less than 25 μm. In the close-packed model, the absorption of laser energy in the powder layer mainly occurs on the surface of the powder layer, and the energy penetration depth is about 30 μm. The energy accumulation in the pores appears at substrate and the depth of one powder particle size, and the energy penetration depth in the loose-random-stacking model is twice the powder particle size, which is the optimal spreading thickness. The absorptivity of the powder layer with a Gaussian particle size distribution and the powder layer with a uniform particle size distribution are 69.2% and 70.3%, respectively, and there is a possibility of mutual substitution between two particle size distributions. The results of this study can provide theoretical basis for the preparation of powder materials in SLM. At the same time, the developed calculation model can be extended to calculate the absorptivity of laser energy in the powder layer composed by other materials.