The structure of natural Ruby was studied by In-situ diffuse reflectance middle infrared spectroscopy.The experiment found that the main infrared absorption modes of ruby include the infrared absorption mode corresponding to the displacement of O2 ? ions in α -Al2O3 molecules towards stationary Al3+ ions (να -Al2O3- -O Al）， the infrared absorption mode corresponding to the displacement of O2 ? ions in Cr2O3 molecules towards stationary Cr3+ ions (νCr2O3- -O Cr）， the infrared absorption mode corresponding to the displacement of O2 ? ions in Fe2O3 molecules towards stationary Fe3+ ions (νFe2O3- -O Fe），and the infrared absorption mode corresponding to the displacement correspondence between Al3+ ions in α -Al2O3 molecules (να -Al2O3 Al Al ）and characteristic infrared absorption mode of gahnite(νZnAl2O4）. Ruby structure mainly includes ：α -Al2O3 ，Cr2O3 ，Fe2O3 ，and a small amount of coexisting gahnite.In situ diffuse reflectance middle infrared spectroscopy is of great value in studying the structure of small Ruby like minerals.