The invention discloses a preparation method of a modified cellulose/polylactic-acid nanometer fiber composite film used for heavy-metal ions to adsorb, which is characterized by including: mixing thermoplastic carboxymethyl cellulose and polylactic acid to obtain mixture according to the weight proportion of 10-90:90-10, adding 10-100 parts of the mixture into 100-500 parts of organic solvent, stirring and dissolving for 0.5-2 hours, shaking ultrasonically for 0.2-1 hour to obtain mixed solution, adding the mixed solution into an electrostatic spinning device, collecting after electrostatic spinning, drying 0.2-2.0 hours at the temperature of 20-45 DEG C to obtain a finished nanometer fiber composite film. The preparation method of the modified cellulose/polylactic-acid nanometer fiber composite film used for heavy-metal ions to adsorb is simple, fast, high in efficiency, low in cost, environment-friendly and the like, and the prepared nanometer fiber composite film can be used in the fields of heavy-metal ions and radionuclide adsorption, separation and recovery and waste treatment, is degradable completely after being used and environment-friendly.