The invention discloses a starch-modified heavy metal trapping agent and a preparing method thereof. The preparing method comprises the steps that firstly, starch is taken and subjected to a cross-linking reaction with a cross-linking agent under the action of a catalyst, and cross-linked starch is obtained; secondly, the cross-linked starch and an etherifying agent are subjected to an etherification reaction under the action of an initiator, and cross-linked etherified starch is obtained; thirdly, in an alkaline environment of a sodium carbonate aqueous solution, the cross-linked etherified starch and an aminating agent are subjected to an amination reaction, and the obtained cross-linked amino starch is the starch-modified heavy metal trapping agent, wherein the content of amino in the modified starch is 1.2-1.8%. Compared with the prior art, the sodium carbonate aqueous solution is used for providing the alkaline environment in the amination reaction, the content of amino of the modified starch can be increased through the preparing method, the starch-modified heavy metal trapping agent has the advantage of being high in copper removing rate, and a chelate formed by the starch-modified heavy metal trapping agent and heavy metal ions is insoluble in water, and can be easily separated from a solution.