Humic acid of high purity is used for the cathode plate expanders of lead storage battery, and also important materials of organic polymer synthesis. This invention relates to a method of extracting and purifying humic acid from low-grade coal. The process is divided into four steps: (1) using alk solution to extract humic acid from coal, adding flocculant, centrifuging to remove dregs to get Humic acid sodium salt, (2) under the meta acidity condition, adding complexing agent to remove high cation, (3) after acidification, using electrodialysis method to elute mineral salts and soluble in the colloidal system, (4) after drying, getting the humic acid powder with purity more than or equal to 95%.