The invention belongs to the technical field of organic photoelectric materials, and particularly relates to transition metal complexes used as phosphorescent materials as well as a preparation methodand application thereof. According to the invention, the complexes prepared by the method not only have high luminous efficiency, but also have adjustable wavelength of emitted light within a wide range. The main ligands of the complexes have the same framework structures, and the method adjust the wavelength of the emitted light in the wide range from red light to near-infrared light through changing the positions of heteroatoms on the main ligands and heteroatom species; and in addition, the preparation methods for different color light emitting materials are basically the same, and the operation is simple.