The invention relates to the field of mineral resource processing technology, in particular to a method of producing high-purity magnesium hydroxide and magnesium oxide by low-grade magnesite. The method comprises the following steps: calcining low-grade magnesite in a calcining furnace at calcining temperature to obtain light-calcining magnesite ore and carbon dioxide, recovering the carbon dioxide, and crushing the calcined light-calcining magnesite ore. By utilizing a method of realizing leaching, purification and separation coupling of the low-grade magnesite and producing high-pruity magnesium hydroxide and magnesium oxide through recycling of ammonia-ammonium slat, the advantages of preparing high-purity products by adopting the traditional bittern-ammonia process can be fully achieved, the method is of green and cycling clean production technique method, no wastes are emitted in the whole process, resources can be comprehensively utilized, the utilization rate of the magnesite resource can be improved, strong acid is not consumed in the technique process, other constitutions are fully utilized, the pollution to the environment can be reduced, and the method provides a new approach for reasonably developing and utilizing the low-grade magnesite resources.