The present invention relates to a method for preparing an optical active layer with 1.about.10 nm distributed silicon quantum dots, it adopts high temperature processing and atmospheric-pressure chemical vapor deposition (APCVD), and directly deposit to form a silicon nitrite substrate containing 1.about.10 nm distributed quantum dots, said distribution profile of quantum dot size from large to small is corresponding to from inner to outer layers of film respectively, and obtain a 400.about.700 nm range of spectrum and white light source under UV photoluminescence or electro-luminescence.