The method for preparing a deep trench uses a dry etching process to form a trench in a silicon substrate, and an etching mixture is then coated on the surface of the silicon substrate and inside the deep trench. A portion of etching mixture is removed from the surface of the silicon substrate and the trench above a predetermined depth from the surface of the substrate, and an etching process is then performed using the etching mixture remaining inside the trench to etch the silicon substrate below the predetermined depth so as to form the deep trench. The etching mixture comprises a conveying solution and an etchant, and the viscosity of the conveying solution is higher than that of the etchant. The conveying solution is spin-on-glass or a photoresist, and the etchant is tetramethylammonium hydroxide, ammonium, or hydrofluoric acid. The volume ratio of the conveying solution and the etchant is preferably between 50:1 and 20:1.