The invention relates to a method for preparing an adsorbent 4-sulfocalix[6]arene-modified magnetic nano Fe3O4 and adsorbing low-concentration uranium with the adsorbent. Modification of magnetic nano Fe3O4 with 4-sulfocalix[6]arene is realized as follows: Fe-O bonds are formed through magnetic nano Fe3O4 and phenolic hydroxyl groups on the lower edge of 4-sulfocalix[6]arene to promote grafting of magnetic nano Fe3O4 and 4-sulfocalix[6]arene, and the purpose of modification is achieved. After magnetic nano Fe3O4 is modified with 4-sulfocalix[6]arene, the hydrophilcity of magnetic nano Fe3O4 is increased, the surface area is increased, and magnetic nano Fe3O4 is can be better bonded with UO2<2+> through a complexing reaction, electrostatic bonding and other reactions. The adsorbent has higher adsorption rate and better adsorption effect than the magnetic nano Fe3O4 adsorbent.