The invention discloses a method for producing light magnesium oxide by using phosphate ore reverse flotation magnesium removal tailings. The method comprises the following steps: dissolving the phosphate ore reverse flotation magnesium removal tailings in a hydrochloric acid solution, adding an oxidizing agent, heating to 50-80 DEG C, adjusting the pH value to be 5-7, and filtering so as to obtain filtrate; and adding ammonia water so as to adjust the pH value to 10-11 and generate precipitates, filtering so as to obtain filter residues, washing, drying and calcining the filter residues so as to obtain the light magnesium oxide powder, wherein the phosphate ore reverse flotation magnesium removal tailings contain P2O5, MgO and CaO, and the solid-to-liquid ratio of the phosphate ore reverse flotation magnesium removal tailings to the hydrochloric acid solution is 1:2-1:6 (g/ml), and the concentration of hydrochloric acid is 10-20%. The method has the beneficial effects that the purpose of comprehensively recycling the phosphate ore mill tailings is achieved, the light magnesium oxide is obtained by using technologies such as hydrochloric acid dissolution and removal of impurities by using the oxidizing agent, and the technologies are simple, and the energy consumption is low.