The invention discloses a preparation method of a modified polymer film material for effectively removing trace heavy metal ions in a water body, which comprises the following steps: 1) carrying out plasma treatment on a polymer film; 2) respectively preparing an initiator solution, a monomer solution, a buffer solution, an activating solution and a polyethylene polyamine water solution; 3) immerging the polymer film in the initiator solution, and carrying out surface activation by ultraviolet irradiation; 4) putting the polymer film into the monomer solution to carry out grafting reaction under ultraviolet irradiation; 5) activating the polymer film in the activating solution; and 6) putting the activated film into a polyethylene polyamine water solution to carry out coupling reaction, thereby preparing the modified film material capable of removing trace heavy metal ions in a water body. In the polymer film modifying process, carboxyl and amino groups are introduced to carry out hydrophilization and endow the modified film with complexing capacity, thereby removing the trace heavy metal ions. The invention has the advantages of simple technique, favorable treatment effect, wide application range, low cost, high unit mass adsorption capacity, time saving, high efficiency and energy saving.