The invention provides a method for preparing high-purity magnesium oxide from sodium carbonate industry steaming ammonia gas and salt preparing bittern. The method includes the technical steps of purifying the bittern, preparing a magnesium oxide precursor, separating a precursor mixed solution, washing and drying the magnesium oxide precursor and calcining the precursor to prepare magnesium oxide. The sodium carbonate industry steaming ammonia gas is introduced into the purified bittern, and the precursor is prepared through a hydrothermal reaction. The precursor and a reaction mother solution are separated to obtain the precursor, the precursor is washed repeatedly and then dried, the dried precursor is placed into a crucible, the crucible is placed in a temperature programming furnace to be calcined to 600 DEG C-900 DEG C, the temperature is kept for 2 h, and high-purity magnesium oxide is obtained after the temperature is reduced to room temperature. High-purity magnesium oxide is an important chemical industrial product and has wide application in the field of chemical engineering, metallurgy, environmental protection and electronics. The magnesium resource in the bittern and the sodium carbonate industry steaming ammonia gas are fully used, the preparation cost of high-purity magnesium oxide is effectively reduced, a new path is provided for preparing high-purity magnesium oxide, and the method has wide application prospects.