Porous silicon with micro-nano sizes has been prepared by the metal- assisted chemical etching approach with Cu nanoparticles as the catalyst agents.This method has the advan tages of low cost and simple operation.The effects of the preparation condition on the morphologies and etch ing depth of the porous structures is discussed,such as the concentration of H₂O₂,th e doping level of silicon substrate, the temperature of etching solution and the etching time.An optimal etching con dition is obtained,the concentration of H₂O_2in the reactant is more than 0.70mol/L for highly dop ed silicon substrate,and more than 0.24mol/L for lightly doped silicon substrate.Nano porous structures with layer thickness about 200nm are obtained after etching for 2h at the temperature of 25℃.The average surface r eflectivity of the prepared nanostructures is under 5% for the wide band.While at the optimal temperature of 50℃,stable structures of micro-nano porous silicon with depth about 14－41μm are achieved after etchin g for about 2－4h.The formation mechanism of the surface morphology of the Si wafers under different reactant co ncentrations can be explained with the hole injection model.The action of Cu and other metals on the etching is studied,and the formation of the pores with cubic cone shape is also analyz ed.