The invention discloses a preparation method and application of a composite modified straw active granule carbon adsorption material. The preparation method of the composite modified straw active granule carbon adsorption material comprises the following steps: 1) mixing straw powder, distilled water, an adhesive and composite minerals, granulating the mixture, and then, putting the granules intoa tube furnace to be pyrolyzed to prepare straw granule carbon; 2) introducing inert gas into a modified reagent, adjusting the pH value of a solution to be 8-11, adding a ferric nitrate solution, andperforming reaction to obtain a composite modified solution; and 3) soaking the straw granule carbon in the composite modified solution for 30min, performing ultrasonic treatment for 1-12h at 1-8MPa,performing centrifugation and drying, putting the dried straw granule carbon into a muffle furnace to be fired for 2-8h at 200 DEG C, and performing cleaning and then drying to obtain the composite modified straw active granule carbon adsorption material. The composite modified straw active granule carbon has a good absorption effect on phosphate radicals in low-pollution water. The adsorption material is convenient to recover, is favorable for recycling of phosphorus resources, and has wide market application prospects.