The invention relates to a flame-retardant antibacterial sound-insulating composite fiberboard and a preparation method thereof, and belongs to the technical field of new materials. The flame-retardant antibacterial sound-insulating composite fiberboard comprises a core layer, sound-absorbing fiber layers, anti-static fiber layers and flame-retardant fiber layers, wherein the core layer adopts fir which is environmentally friendly, has antibacterial health-care effect and has natural original wood fragrance, and the sound-absorbing fiber layers, the anti-static fiber layers and the flame-retardant fiber layers are arranged on the two sides of the fir, so that the composite fiberboard has excellent flame-retardant, anti-static and sound-insulating effects, the risk of fire combustion and harm of static electricity on the health of a human body are reduced, and the use safety performance is improved; meanwhile, a nano-graphene layer has high heat conductivity, so the overall mechanical strength is improved, the antibacterial property is achieved and the antibacterial property of the composite fiberboard is improved; the composite fiberboard adopts graded pavement, so the strength and the impact resistance of the composite fiberboard are improved; the composite fiberboard does not contain formaldehyde and is environmentally friendly.