In the paper, the effects of porosities and average particle sizes of powder layer on light absorption during SLM process were investigated, in which closed-packing models based on Horsfieled’s filling method were established and light absorption was simulated using ray tracing based on laser-material interaction mechanism. The results show the absorption of powder layer of Ti6Al4V alloy is higher than 70%. The decrease of porosity of powder layer benefits to improve the absorption, while the absorption tends to decrease if porosity decreases to a certain value due to the reflection. The decrease of average particle size of powder particles benefits also to improve the absorption. If the light irradiates at positions with different particle arrangements, the absorption behavior changes with irradiation condition whether there occurs the multi-reflection. The above research provides theoretical basis for preparation of new powder materials, their parameter developing for SLM technology and even the properties regulation of SLM fabricated component.