A novel silicon-based micron/nanometer structural composite system, silicon nanoporous pillar array(Si-NPA), was prepared on the substrate of single-crystal si licon (sc-Si) wafers by a hydrothermal etching method; and the studies on its mo rphological structural and optical properties were carried out. Structural exper iments disclose that Si-NPA could be well described by triple hierarchical struc tures: the array composed of micron-sized silicon pillars, the nanopores densely distributing on each pillar, and the silicon nanocrystallites constructing the walls of nanopores. Optical measurements prove that Si-NPA has good performances on light absorption and photoluminescence(PL). Based on the experimental data o f the integral reflectance spectrum, the structural and optical parameters such as complex refractive index, complex dielectric constant and absorption coeffici ent of Si-NPA are calculated by adopting Kramers-Kronig transformation; based on which, the origin of the notable difference between the optical properties of S i-NPA and sc-Si is discussed. Through analyzing the function relation between th e absorption coefficient of Si-NPA and the photon energy of incident light, the characteristic of the electronic structure of Si-NPA is proved to be that of a d irect-band-gap semiconductor, and the calculated bandgaps agree well with the PL peak energies given by experiments.