The absorption characteristics of micro-structural material, such as “black silicon”, aroused extensive attention. In order to understand the impact of the micro-structure, based on Monte Carlo method, the optical simulation is used for absorption characteristics of micro-structural material. In this paper, the body material used for simulation is a kind of glass, BK7, which has been widely used. And, there are a number of 400 structual cells in the model. The bottom square of each cell is fixed to 50×50 µm, and the height of cells is adjustable. For each height, the reflectance, transmittance and absorption of microstructural material can be caculated. Then the relationship between the height and reflectance, transmittance and absorption can be built up. In our simulation, a very complex relationship between the absorption and the the structure of the material is showed. Generally, the absorption increases while the height of microstructual cell increasing.