A method for preparing a -Ga.sub.2O.sub.3 nanomaterial, comprising a step of treating a mixture comprising a gallium element, water, and an organic solvent with ultrasound. The preparation process and equipment requirements are simple, the cost of materials is low, there are fewer experimental parameters, and experimental conditions are mild, with no additional heat source and/or pressure being applied. The -Ga.sub.2O.sub.3 nanomaterial can be prepared, in kilograms or above, quickly at an ambient temperature and pressure.