The invention provides a preparation method of a wetland plant and pig manure composite biochar, and an application of the composite biochar in the adsorption of heavy metals in water. Typical wetlandplants of Poyang Lake are selected, the aerial parts of the wetland plants are harvested in the growing season, pig manure is obtained from a pig farm, the collected wetland plant biomass is washed with water, dried and crushed, the pig manure is dried and crushed sequentially, and the plant biomass and pig manure biomass are mixed according to a ratio 1:1 to form a mixed biomass; chitosan and alkali residues are dried, and are crushed and mixed according to a mass ratio of 1:1; the mixed biomass and the obtained chitosan and alkali residue powder are mixed according to a ratio of 10:1 to prepare a composite biomass; and the composite biomass is added to an iron solution, anaerobic hydrothermal carbonization is carried out to form the composite biochar, and the composite biochar is used for adsorbing heavy metals in water. The preparation method fully utilizes several typical wastes in the Poyang Lake basin, achieves the purpose of treating pollution with wastes, and also has the advantages of wide sources of raw materials, low cost, simplicity, and easiness in operation, and the prepared biochar adsorbent is nontoxic and harmless, and is friendly to the environment.