Tetragonal germanium dioxide has excellent electrical and optical properties and holds significant application value in fields such as electronic components and others. However, its large-scale preparation faces several challenges. Most of the existing methods for converting hexagonal germanium dioxide to tetragonal germanium dioxide require medium-to-high temperature conditions, high-pressure environments, and catalyst assistance, and generally have problems such as long reaction cycles and easy introduction of impurities. In this study, we developed a novel method for preparing pure tetragonal germanium dioxide under the relatively low temperature condition and have thoroughly investigated effects of temperature, pressure, and oxygen fugacity on the product formation. The research results indicate that hexagonal germanium oxide can be completely converted to tetragonal germanium oxide by adding gaseous water under conditions of temperatures ranging from 200 °C to 450 °C, and low pressure of 1.30×10³ kPa. Raman spectroscopy and X-ray diffraction analyses confirmed that the experimental product possesses a high-purity tetragonal structure with no detectable impurities. Comparative experiments using different oxygen fugacity buffer pairs (Ni-NiO, Fe₃O₄-Fe₂O₃, MoO₂-MoO₃) demonstrated that the tetragonal germanium oxides which were produced under the oxygen fugacity conditions of over 1.05×10^–28 kPa have good thermodynamic stability. Compared to previously reported preparation methods of the tetragonal germanium dioxide, this synthesis route has three main advantages including the easily achievable reaction conditions, no catalyst requirement, and a straightforward process flow. By increasing the reaction vessel capacity, the batch conversion from hexagonal germanium oxide to tetragonal germanium dioxide can be accomplished. This novel preparation method offers a viable solution for the large-scale production of high-quality tetragonal germanium oxide, which is conducive to promoting applications of the tetragonal germanium oxide in the fields of new electronics and optoelectronics. This study optimized the preparation process of tetragonal germanium oxide, greatly enhancing the application potential of tetragonal germanium oxide in high-tech sectors and providing crucial support for the development of related industries.