The invention belongs to the technical field of treatment and utilization of three wastes in the chemical industry, and particularly discloses a method for preparing magnesium carbonate to co-produce calcium carbonate and sodium sulfate by using pure alkali waste salt slurry. The invention has the main technical scheme that: the pure alkali waste salt sludge is washed, settled and filtered, and filtered liquor enters a carbonizer for carbonization, and is pumped and filtered, then heated and decomposed, precipitated and centrifuged to obtain light magnesium carbonate; the precipitated filter cake is stirred into emulsion and then reacts with calcium carbonate, the reactant is pumped and filtered, and the obtained filter cake is cleaned, dried, crushed and packaged to obtain a calcium carbonate product; and filtered liquor is distilled, cooled and crushed to obtain a powdery sodium sulfate product. In the method, not only is salt slurry of the pure alkali industry effectively treated, chemical pollution is eliminated, but also the waste salt slurry is modified to have a light magnesium carbonate product, the calcium carbonate product and the sodium sulfate product with economic values, the production technology and production equipment are simple and the cost is low.