The invention discloses a damp-proof building material for an archive room and a preparation method thereof. The damp-proof building material comprises a plate body and a waterproof layer painted on the bottom surface of the plate body. The waterproof layer comprises the compositions of acrylic emulsion, water, a processing modifier, a dispersing agent, a lubricating agent, an antifoaming agent and a filler. The preparation method comprises the steps that a, a waterproof layer coating is produced; b, a waterproof layer painting device is mounted; c, a painting area is divided; d, plate body painting is prepared; e, the painted surface of the plate body is painted; and f, the waterproof layer is subjected to solidification curing. According to the damp-proof building material for the archive room and the preparation method, the waterproof coating has high elasticity, can resist slight vibration of the building structure, is high in adhesive property and good in damp-proof performance, and can be directly constructed on a damp base surface; and in the preparation process, the waterproof layer coating is evenly painted in a layered mode on the surface of the plate body, the thicknessof the waterproof layer is strictly controlled, the crack phenomenon of the waterproof layer after painting is completed can be reduced, the painting quality of the waterproof layer is improved, and damp-proof performance of the damp-proof building material is enhanced.