In this work, A novel two-step metal assisted chemical etching (MACE) method with A g and Cu dual elements as catalysts at the temperature of 25℃ and 50℃ was introduced to prepare nano -structure on monocrystalline silicon.The effects of etching time and molar ratio of Ag/Cu on the surface mor phology and reflectance of black silicon were systematically studied.It was found that the black silicon by Ag/C u-assisted etching possessed lower surface reflectance compared with the black silicon by Ag-assisted or Cu-assis ted etching.The black silicon has smoother surface and more complex structure with lateral and vertical holes by A g/Cu-assisted etching compared with that by Ag or Cu-assisted etching method.The complex structure possessed a good light trapping effect.A minimum surface reflectance of lower than 4% in the wavelength range from 300nm to 1000nm was obtained when Ag/Cu molar ratio was 1/5and etching time was 10mins.While for the high dopin g silicon substrate,the holes on the surface of black silicon obtained by Ag/Cu-assisted etching were uniform an d regular inverted pyramid structure,the reflectivity was less than 2%.