an soi wafer and a method for forming the same , where the method for forming an soi wafer includes : preparing a monocrystalline silicon wafer on which a mask layer is formed ; etching the mask layer and the monocrystalline silicon wafer to form several trenches ; forming a first insulating layer on the sidewalls and the bottoms of the trenches ; etching and removing the first insulating layer on the bottoms of the trenches ; etching along the trenches the monocrystalline silicon wafer beneath the trenches to form cavities ; processing the inner walls of the cavities to form a second insulating layer ; and filling up the trenches and the cavities with an insulating material layer . the process of the invention is easy to be implemented at a low manufacturing cost and an soi wafer being formed is of high quality while being capable of being compatible with a standard process of manufacturing a bulk silicon cmos .