The invention discloses a modified cellulose chelating agent preparation method, which comprises: dissolving etherified cellulose in water, and adjusting the pH value to 12-13 with an alkali solution;sequentially adding diethylenetriamine and formaldehyde, carrying out a reaction at a temperature of 85-95 DEG C, cooling to a room temperature after completing the reaction, adding carbon disulfide,and continuously carrying out the reaction at a room temperature; and separating the reaction product, washing, and drying to obtain the modified cellulose chelating agent. According to the present invention, cellulose used as the raw material is subjected to a series of reactions to modify into the capturing agent having the chelating group, and the capturing agent having the chelating group andheavy metal ions form chelating precipitates; and the chelating agent has characteristics of good adsorption effect, good selectivity, easy elution, easy stability, easy storage of precipitated residue, and the like.