The application discloses a high-whiteness MGO substrate, a preparation method thereof and a decorative board having the substrate. The high-whiteness MGO substrate includes a surface layer and a substrate, wherein the substrate is prepared from a forming agent, a lightweight filler, a modifier and water in parts by mass as follows: 40-49 parts of light burned magnesium oxide powder, 18-25 parts of magnesium sulfate heptahydrate, 16-25 parts of a polyvinyl alcohol solution, 16-20 parts of a plant powder, and 0.5-2 parts of a modifier; the modifier being obtained by mixing citric acid, phosphoric acid, and sodium sulfate in a mass ratio of 10:3:6.