The invention belongs to the technical field of multifunctional foliage fertilizer and provides a method for preparing chelating magnesium. The preparation steps are as follows: preparing 20% of magnesium carbonate suspension first by volume percent, and adding the 20% of magnesium carbonate suspension during the preparation process of compound microbial activated solution; and the specific method comprises the steps of combining the following materials by volume ratio: 3 % to 5% of EM stock solution, 2% to 5% of honey, 25% to 30% of the magnesium carbonate suspension and 60% to 70% of deionized water, blending the mixture uniformly, and putting the mixture in a rotary shaking table with the speed of 120 r/min to 180 r/min and fermenting for 6 to 7 days under ambient temperature. Therefore, chelating magnesium can be obtained. The method has the advantages that the magnesium fermented with compound microbe is in a chelating (complexation) state or an anion cation balance state, is easily insoluble in water, and is easy to be absorbed by plants; and when the chelating magnesium is applied on leaves, the chlorophyl content of vegetables can be improved, and the problem of physiological barrier of plants caused by lack of magnesium can be solved effectively.