The invention discloses a halogen-free flame-retardant composite material which comprises the following components in parts by weight: 25-40 parts of modified rice hull powder, 27-46 parts of modified bagasse, 10-20 parts of p-diacetylene-benzene, 5-10 parts of triethyl borate, 80-100 parts of organic silicone oxygen resin, 10-20 parts of magnesium hydrate, 10-20 parts of aluminum hydroxide, 1-4 parts of a coupling agent and the like. A preparation method comprises the following steps: (1) weighing p-diacetylene-benzene, triethyl borate and organic silicone oxygen resin according to the formula, and melting; (2) mixing a polyethynylbenzene polymer with magnesium hydrate, aluminum hydroxide, an antioxidant and the coupling agent, melting in a mixing mill, putting polytetrafluoroethylene into the mixing mill, and melting; and (3) putting a granular material and a lubricant into a press vulcanizer, and performing hot-pressing at 8-15MPa, thereby obtaining the halogen-free flame-retardant composite material. As the organic silicon oxygen resin is generated, the flame-retardant effect and mechanical properties of the composite material can be improved.