The invention relates to a method for preparing a lightweight microporous magnesium raw material, and belongs to the technical field of refractory materials. The steps and parameters are as follows: preparingcaustic-burned magnesia powder with granularity smaller than 0.080mm, magnesite powder granularity not greater than 0.080mm and pulp waste raw material with specific gravity of 1.0-1.6g/cm3; performing ball grinding on 5-95wt% of caustic-burned magnesia powder and 5-95wt% of magnesite powder for 1-5h in a ball mill, and mixing the mixture with the pulp waste accounting for 3-8% of the total weight; pressing green bricks by using a press machine; and drying, sintering and cooling to the room temperature, and discharging the finished products from the kiln. The lightweight microporous magnesium raw material has the advantages of having good mechanical properties under normal temperature and high temperature while on the premise of ensuring no obvious reduction of the alkali erosion resistance of the material; reducing the load of the cylinder body while reducing the temperature of the cylinder body of the high-temperature kiln, and thus realizing the purposes of saving energy and reducing the consumption.