AbstractNatural fiber is a renewable resource characterized by its low cost and environmental friendliness. However, flame retardant properties are one of the biggest limitations for the preparation of composite materials that need to be improved. In this work, a novel complex flame retardant consisting of aluminum hydroxide (ALH) and decabromine diphenyl oxide (PBDE) was proposed to inhibit the thermal decomposition. Flame-retarding paper was made from softwood pulp and complex flame retardant. The thermal properties of the flame retardants were studied using thermogravimetric analysis (TGA). The mechanical properties of paper treated at different temperatures were tested, while the surface characteristics of natural fiber were detected by a scanning electron microscope (SEM) and atomic force microscope (AFM); their specific surface areas were also measured. The optimum value of aluminum hydroxide to decabromine diphenyl oxide was 3 to 1, added at 70% based on dried natural fiber. It also had good flame retardant performance and mechanical properties at 200 °C for 5 min; meanwhile, the tensile index of the handsheet was 82.5 Nm/g, and the specific surface area was 0.414 m2/g.