Disclosed is a method for preparing polycrystalline aluminum oxynitride having enhanced transparency, and preparing polycrystalline aluminum oxynitride by sintering a powder mixture of Al2O3 and AlN under atmospheric pressure, wherein the content of AlN is set to 17 to 26 mol %, a first sintering is performed at 1,575° C. to 1,675° C. so as to enable raw-material powders to have a relative density of 95% or higher, and a second sintering is performed so as to enable the raw-material powders to have a higher relative density.