The invention relates to a method for producing a flexible building decorative board. The method comprises the steps that a substrate is prepared, wherein plaster, sodium trimetaphosphate, corn starch and water are mixed and divided into slurry A and slurry B; glass fiber and antifoaming agents are added into the slurry A and stirred uniformly; compressed air is added into the slurry B; the slurry A is poured into a formwork, after the slurry A is struck off, the slurry B is poured into the formwork, after the slurry is dried in a stationary mode, the substrate is obtained; bonding agents are prepared; ethylene propylene rubber powder, polypropylene, light calcium carbonate, antioxygen and coloring agents are added into a sealed type high-speed mixer in proportion, mixed and stirred at a fast speed; the raw materials which are processed ready are guided into an extruder, blended, kneaded and squeezed out; the bonding agents are poured into the surface of the substrate, after the bonding agents are struck off, a flexible layer is laid with a metallized film facing downward, and the decorative board is obtained after cooling. The decorative board has the advantages of being high in strength, long in service life, high temperature resistant, low temperature resistant and the like.