Due to the high absorption efficiency in both visible and infrared wave bands, micro/nano-structure silicon prepared by femtosecond laser has many important applications in the silicon-based optoelectronics, photoelectric detectors and super hydrophobic devices. However, the mechanism of the absorption characteristics with wide spectrum and high efficiency has never been quantified accurately, which limits further development and application of this kind of material. Therefore, we experimentally quantified the different absorption factors of the micro/nano-structure silicon prepared by femtosecond laser, including the dopant impurities in the silicon substrate, doping impurities induced during the laser fabrication process, absorption enhancement from the surface amorphous light-trapping structure. By these analyses, we determine that as compared to the doping sulfur impurities induced during the fabrication process, dopant impurities in the silicon substrate can contribute much more to the infrared absorption. Furthermore, the material is annealing-insensitive. These results have important influence on the design and manufacture of high efficiency optoelectronic devices.