The present invention is based on the characteristic of hot concentrated ammonium alum solution that it does not crystallize under the conditions of being shielded from light and left stand still. Starting from aluminum compounds, aluminum salt is formed. The latter reacts with ammonium salt, sulfate and sodium salt (particularly sodium sulfate) to form ammonium alum and sodium nitrate. By crystallizing selectively under either exposure to light or being shielded from light and left stand still, and controlling the concentration for crystallization, separation of ammonium alum and sodium nitrate crystals can be effected, so that the product sodium nitrate and by-product ammonium alum are finally obtained. This method provides a new approach for expanding the production of sodium nitrate. Besides, production cost is low, the raw materials are abundant and the equipment is easy to procure. It is suitable for industrial production on large or small scale.