The invention provides a flame retardant composite board. The flame retardant composite board comprises a homogeneous naked panel and a facing layer arranged on at least one surface of the homogeneous naked panel, wherein the homogeneous naked panel is prepared from raw materials of, based on weight parts, 65-98.5 parts of plant straw particles, 3-10 parts of binder and 3-10 parts of fire retardant, wherein the fire retardant is prepared from raw materials of magnesium oxide, calcium sulfate, aluminum hydroxide, brucite, zinc molybdate and zinc borate. The flame retardant composite board has good chemical properties of fire retarding, water prevention, insect prevention and the like, and good physical and mechanical properties, can replace wood artificial boards, and is used in various fields. The invention further provides a manufacture method of the flame retardant composite board. The manufacture method of the flame retardant composite board mainly adopts a normal temperature cold press molding technology to solidify and shape the raw materials of the naked panel, not only greatly saves energy, but also improves product performance, enables the flame retardant composite board to have water prevention and fire prevention functions, and simultaneously can substantially reduce equipment cost, achieves a whole green production process, and does not cause pollution during the whole production process.