We present a new method in which both positive and negative pulses are used to etch silicon for fabrication of porous silicon (PS) monolayer. The optical thickness and morphology of PS monolayer fabricated with different negative pulse voltages are investigated by means of reflectance spectra,scanning electron microscopy and photoluminescence spectra. It is found that with this method the PS monolayer is thicker and more uniform. The micropores also appear to be more regular than those made by common positive pulse etching. This phenomenon is attributed to the vertical etching effect of the PS monolayer being strengthened while lateral etching process is restrained. The explanation we propose is that negative pulse can help the hydrogen cations (H+) in the electrolyte move into the micropores of PS monolayer. These H+ ions combine with the Si atoms on the wall of new-formed micropores, leading to formation of Si-H bonds. The formation of Si-H bonds results in a hole depletion layer near the micropore wall surface,which decreases hole density on the surface,preventing the micropore wall from being eroded laterally by F- anions. Therefore during the positive pulse period the etching reaction occurs exclusively only at the bottom of the micropores where lots of holes are provided by the anode.