Disclosed in the present invention is a benzocrown ether graft polymer with a lithium isotopic separation effect and a preparation method thereof. The polymer is a benzocrown ether graft polymer formed by the linkage of chemical bonds, which takes the main chain of a polymer containing chloromethyl group, chloroformyl group or hydroxyl group as main chain, and takes a benzocrown ether as pendant group. The preparation process of the polymer comprises the following steps: preparing polymer solution with certain concentration by dissolving a polymer containing chloromethyl group, chloroformyl group or hydroxyl group in a solvent; then blending a catalyst and a benzocrown ether containing carboxyl group or aldehydyl group and dissolving in the polymer solution containing hydroxyl group, or blending an acid-binding agent and a benzocrown ether containing amino group or hydroxyl group and dissolving in the polymer solution containing chloromethyl group or chloroformyl group, reacting at a certain temperature and for a certain time, linking the benzocrown ether to the main chain of polymer by chemical bonds, and precipitating by adding a precipitating agent to obtain the graft polymer. The grafting polymer has excellent characteristic of lithium isotopic separation.