The invention relates to the field of 
semiconductor preparation, in particular to a method for preparing an ONO medium layer. The method is suitable for a nonvolatile NOR flash memorizer. The method includes the following steps of firstly, providing a 
silicon wafer, wherein a surface tunneling 
oxide layer and a floating gate layer formed at a preset position are sequentially arranged on a substrate of the 
silicon wafer; secondly, inputting first preset reaction gas through an in-situ vapor 
generation process, and forming bottom layer 
silicon oxide on the upper face of the floating gate layer; thirdly, inputting second preset reaction gas through a porous 
quartz tube, and forming 
silicon nitride on the upper side of the bottom layer 
silicon oxide through deposition; fourthly, inputting the first preset reaction gas through the in-situ vapor 
generation process, and oxidizing the surface of the 
silicon nitride to form top layer 
silicon oxide. The method has the advantages that the thickness uniformity of a thin film depositing on the surface of the silicon 
wafer is effectively improved, shells produced by the silicon wafer is remarkably reduced, N type dopes are not prone to being formed on contact faces of the 
silicon oxide and the silicon wafer, 
realizability is high, and the method can be widely applied to various deposition processes.