The invention relates to the field of inorganic and non-metal material preparation, in particular discloses a method of using magnesite or brucite as raw material to prepare magnesia whiskers by technologies of calcinations, fine grinding, hydration, carbonization, solid-liquid separation, dilution, pyrolysis, filtration, drying and calcination. The method comprises the following steps: adding 0.1 percent to 2.0 percent of soluble magnesium salt to high-magnesian water with the concentration of 3-100kg/1000L to be pyrolyzed at a temperature of 40 DEG C to 80 DEG C under the stirring intensity of 50-500 round/min; firstly, preparing precursor magnesium carbonate whiskers; further calcining the whiskers at a temperature of 500 DEG C to 800 DEG C to obtain magnesia whisker products which have favorable crystallization growth, high purity and slenderness ratio and good dispersion and can be applied to the fields of material, chemical engineering, machinery, and the like, and the precursor magnesium carbonate whiskers can be also applied to the fields of material, chemical engineering, and the like and the preparation of magnesium hydrate, bitter salt, magnesium chloride, magnesium nitrate and other magnesium salt products. The method has simple technology, little investment and low production cost, has no pollution in a production process and can utilize various grades of magnesite and brucite lump ores and powder (tailing) to prepare the magnesia whiskers.