The black silicon was prepared by non-mask reactive ion etching. The effects of microstructure density and height on the optical properties of black silicon were studied by means of scan-electron microscopy and UV-VIS-NIR spectrophotometer. The results show that, the higher the height and the greater the microstructure density of the microstructure of black silicon, the greater the absorptance. The higher height of microstructure is more beneficial for the absorption of near infrared light. The absorptance of black silicon prepared by non-mask reactive ion etching remains stable under high temperature annealing from the visible (400 nm) to near infrared (1 700 nm) wavelength region.