The invention discloses a diatomite decorative acoustic board, which belongs to the field of building material. The invention solves the problems that the prior porous acoustic material has low acoustical absorptivity to middle and low frequencies, a mineral wool acoustic board pollutes the environment and so on. The board consists of 70 to 80 weight percent of diatomite, 18 to 25 weight percent of gypsum, 1 to 3 weight percent of pigment, 0.9 to 1.5 weight percent of adhesive, and 0.1 to 0.5 weight percent of fiber; the content of SiO2 in the diatomite is more than 85 percent, and the white content is more than 82; and the white content of the gypsum is more than 90 degrees. The method for preparing the board comprises the following steps: solid materials, namely the diatomite, the gypsum, the adhesive, the pigment and the fiber are added into a mortar mixer to be stirred for 20 to 30min, water which accounts for 1 to 1.5 times of the amount of the solid materials is added into the mixture after the mixture is stirred evenly to be prepared into uniform slurry, the slurry is cast into board molds with different specifications, and the mold stripping and drying are performed after 24h of maintenance. The board has functions of sound absorption and sound insulation, humidity conditioning and dewing prevention, fire resistance and flame retardation, thermal insulation, environmental protection, beautiful decoration, high strength, light mass, easy installation, convenient maintenance and so on, and is suitable for mass production.