The invention belongs to the technical field of building materials, and specifically relates to a magnesium oxysulfate plate and a preparation method thereof. The magnesium oxysulfate plate comprises the following components by weight: 200 to 300 parts of magnesium oxysulfate cement, 10 to 20 parts of alkali-resistant glass fiber, 30 to 50 parts of aggregate, 10 to 50 parts of water, 5 to 10 parts of an early-strength agent, 5 to 10 parts of a water reducing agent and 20 to 80 parts of a reinforcing material. The magnesium oxysulfate plate provided by the invention has low cost, is greatly improved in the aspects of strength and water resistance, has the advantages of light weight, low alkali, water resistance, high-temperature resistance, non-corrosion steel bar, etc., and is applicable to manufacturing of fireproof plates, refractory decorative materials, etc.