The invention discloses a high-strength gypsum board and a preparation method thereof, and relates to the technical field of novel building materials. The high-strength gypsum board comprises the following raw materials: modified gypsum powder, Zuoyun clay, anorthite powder, nano tourmaline powder, coarse whiting powder, borax, a silicon carbide fiber, p-hydroxybenzoic acid isopropyl ester, an organic silicon emulsion, a coupling agent, an anti-mold agent, a defoaming agent and an adhesive. The preparation method comprises the following steps: mixing raw materials, aging, performing injection molding, performing natural maintenance, demolding, drying, and shaping, thereby obtaining the high-strength gypsum board. The high-strength gypsum board is simple to prepare, and low in cost; and the problem that the conventional gypsum board is low in mechanical strength is solved, that is, the strength of the gypsum board is not greatly affected or even degraded even when the gypsum board is used in moisture and wet environments for a long time, so that the high-strength gypsum board is long in service life, safe, reliable and wide in market prospect.