Abstract: A method for preparing a GaN substrate material includes: performing in-situ epitaxy on a Ga2O3 thin film and a GaN thin film in a multifunctional HVPE growth system. First, the Ga2O3 thin film is grown on a substrate such as sapphire by an HVPE-like method, and the Ga2O3 is nitrided in an ammonia gas atmosphere to form a GaN/Ga2O3 composite thin film. Then, a GaN thick film is grown on the GaN/Ga2O3 composite thin film by HVPE to obtain a high-quality GaN thick film material. The Ga2O3 layer is removed by chemical etching to obtain a self-supporting GaN substrate material. Or, the conventional laser lift-off method is used to separate the GaN thick film from the heterogeneous substrate such as the sapphire to obtain a GaN self-supporting substrate material.