In order to study the special optical absorption efficiency of “black silicon” materials, “black silicon” material with different heights of micro-cones is successfully fabricated, and the geometric parameters of the micro-cones are measured by scanning electron microscope (SEM). Then a model is built to calculate the surface effective absorption area of “black silicon” material based on the measured structure of “black silicon” material, and the surface area of “black silicon” is more than 20 times than that of normal silicon. Meantime, according to the geometrical optics method, the reflectivity is simulated during the wavelength from 200 nm to 2000 nm. The simulation results are relatively close to the experimental results. It is theoretically verified that the strong optical absorption ability of the silicon with micro-nano structure is due to special structure for light trapping effect and the increase of surface effective absorption area.