The invention relates to a preparation method of polyaniline-loaded gamma-PGA (Propylene Glycol Alginate) nano-hydrogel. The preparation method comprises the following steps: carrying out double emulsification reaction on EDC (Dichloroethane)-activated gamma-PGA to obtain a W/O/W polymer emulsion; dissolving cystamine dihydrochloride into ultrapure water, then dropwise adding a mixed solution into the W/O/W polymer emulsion, and reacting to obtain gamma-PGA/Cys nano-hydrogel; dissolving the gamma-PGA/Cys nano-hydrogel into ultrapure water with a pH value of 3.00, then adding the cystamine dihydrochloride, standing in an ice bath for 44 minutes to 1 hour, then adding ammonium persulfate for reacting, dialyzing and purifying to obtain a finished product. The preparation method disclosed by the invention has the advantages of simple process, easiness in operation and separation, wide source of raw materials and low cost; the prepared gamma-PGA hydrogel is uniform in particle size distribution and has the advantages of good water solubility, colloidal stability, biocompatibility, no adverse influence on a living body and potential application value in the field of photothermal therapy.