The invention belongs to the technical field of building materials, and provides an environmental-friendly coating for reducing volatilization of harmful gases and a preparation method of the environmental-friendly coating. The environmental-friendly coating for reducing volatilization of harmful gases comprises the following raw materials in parts by weight: 70-86 parts of a polyurethane resin, 3-6 parts of a coloring agent, 0.08-7 parts of a dispersing agent, 1-11 parts of white cement, 1-6 parts of quartz sand, 1-3 parts of cellulose ether, 2-9 parts of medical stone powder, 0.58-3.37 parts of charcoal dust, 4-6 parts of ash calcium and 1-2 parts of packing. According to the environmental-friendly coating for reducing volatilization of harmful gases, attapulgite clay and magnesium slag are added into the formula of the coating, magnesium slag can immobilize carbon dioxide and adsorb harmful gases, the situation that piled magnesium slag occupies a great amount of farmlands is effectively avoided, the problem that the environment can be polluted by the magnesium slag is reduced, and the production cost can be greatly reduced when the magnesium slag is adopted to prepare coatings.