The invention belongs to the technical field of composite materials, and relates to a preparation method of a modified biomass charcoal composite chitosan-based adsorbent, which comprises the following steps: weighing potassium hydroxide, dissolving in deionized water, placing in an ice bath, dropwise adding acrylic acid for neutralization, adding modified biomass charcoal when the neutralization degree reaches 60-80%, and ultrasonically stirring uniformly to obtain a solution A; ultrasonically dissolving chitosan and disodium ethylene diamine tetraacetate in deionized water, and obtaining a solution B; uniformly mixing potassium persulfate and N'N-methylene bisacrylamide according to a volume ratio of 1: 1 to obtain a solution C, adding potassium persulfate serving as an initiator and N'N-methylene bisacrylamide serving as a cross-linking agent, carrying out ultrasonic treatment for 1 hour, reacting at 70 DEG C for 2 hours to obtain a hydrogel crude sample, soaking and washing with absolute ethyl alcohol for multiple times, carrying out vacuum drying at 70 DEG C for 24 hours, grinding into powder, drying and storing. The prepared adsorbent can be applied to adsorption of pollutants in water. The raw materials are low in cost and easy to obtain, the synthesis process is simple, the adsorption performance of the hydrogel is improved, waste is turned into wealth, and the product has a great application prospect.