The present invention discloses a phosphate ester fire retardation agent prepared through a residual oil modification reaction, and a method for preparing a fire retardation polyolefin composite material by adopting an intumescent fire retardation agent prepared through compounding of the phosphate ester fire retardation agent. According to the method, aromatic distillate is separated from residual oil, an organic phosphorus compound grafting reaction is performed to prepare a residual oil modified fire retardation agent, 10-30 parts by mass of the residual oil modified fire retardation agent, 30-50 parts by mass of ammonium polyphosphate, and 10-30 parts by mass of ammonium polyphosphate melamine are mixed, and the obtained mixture, 1-5 parts by mass of a synergist and 1-5 parts by mass of an antioxidant are subjected to a blending and melting composite process to prepare the intumescent fire retardation agent. According to the present invention, the residual oil modified fire retardation agent component is adopted as the charcoal source and the acid source, and the residual oil modified fire retardation agent component, the ammonium polyphosphate (acid source, gas source), and the ammonium polyphosphate melamine (air source) form the intumescent fire retardation agent so as to provide characteristics of no corrosive gas, low smoke, low toxicity, and good fire retardation; 10-35 parts by mass of the fire retardation agent composition is added to 90-65 parts by mass of the polyolefin, such that the fire retardation is significantly improved, and the mechanical property is not affected; and with the residual oil modification approach, the high added value utilization of the residual oil is achieved, and the new rich source is provided for the fire retardation agent.