The invention relates to the field of extraction of uranium from seawater, and particularly provides a method for preparing plant-inorganic composite absorbents from phosphoric-acid-modified folium cycas for extracting uranium from seawater.The method includes drying and smashing folium cycas leaves; modifying the folium cycas leaves by the aid of phosphoric acid to obtain products; washing, drying and activating the products to obtain the biological adsorbents for extracting the uranium from the seawater.The method has the advantages that the plant-inorganic composite adsorbents are high in adsorption efficiency when applied to adsorbing the uranium from the simulated seawater under the conditions that a pH (potential of hydrogen) value is 8.7, the temperature is 25 DEG C and the adsorption equilibrium time is 240 min; the adsorbents can be reused after the uranium is adsorbed and the adsorbents are desorbed by the aid of 0.08 mol/L nitric acid, and the high adsorption efficiency can be kept; the adsorbents are made of materials from wide sources and are easy to prepare and desorb, low in cost, high in mechanical strength and little in environmental pollution, good adsorption effects can be realized, and the like.