The invention discloses a water-resistant magnesium oxysulfate fire-proof plate and a manufacturing method thereof and belongs to the field of fire-proof plate production. The fire-proof plate is prepared by adding active filler into slurry, mixing, hardening under a certain condition and demoulding, wherein modified magnesium oxysulfate is taken as a base material and medium-alkali glass fiber and non-woven fabrics and other materials are taken as a skeleton. Compared with the prior art, the fire-proof door core plate disclosed by the invention has no moisture absorption, higher durability than a conventional glass magnesium plate and smooth and beautiful appearance. The fire-proof plate can be applied to the field of building fire protection, especially fire protection engineering under a humid environment and has a good popularization and application value.