The invention belongs to the technical field of treatment and utilization of waste water, waste gases and waste residues in the chemical industry, and in particular discloses a method for preparing magnesium carbonate and co-producing sodium carbonate and calcium sulfate from soda ash salt mud. The main technical scheme of the invention is that: the soda ash salt mud is washed, settled and filtered, filter liquor enters a carbonizer for carbonization, and the carbonized filter liquor is subjected to filtration, pyrolysis, precipitation, and centrifugation to form the magnesium carbonate; a filter cake is blended into emulsion to react with sodium sulfate, and the product is filtered to obtain a filter cake, the filter cake is washed, dried, crushed and packaged to form the calcium sulfatedihydrate product; and the filter liquor obtained by secondary filtration is distilled, cooled and crushed to form the powdery sodium carbonate product. The method effectively treats the salt mud in the soda ash industry, eliminates the chemical pollution, and can transform the waste salt mud into the magnesium carbonate, sodium carbonate and calcium sulfate products with economic values, and hassimple production technology and equipment thereof, and low cost.