The material black silicon formed by arrays of sharp conical spikes on the silicon surface was fabricated under the cumulative femtosecond laser pulses irradiation in different ambient atmospheres. Using light transmission theory of geometrical optics, the light trapping properties of black silicon and effect of the micron cone-shaped peaks structure shape and density of black silicon surface on the number of reflections were studied. Then a conclusion is drawn from the black silicon surface structure, the higher height, the smaller distance and the greater bottom corner, the better its light trapping effect.