The present invention provides a preparation method of a sandwich-structured nano-adsorbent of carbon-coated magnesium oxide. The method comprises the following steps: (1) adding magnesium chloride hexahydrate, dextrose monohydrate, and a carbonaceous material to ethanol water solution, and stirring the mixture well, wherein the carbonaceous material is at least one of hexadecyl trimethyl ammonium bromide, polyethylene glycol, ethylenediamine tetraacetic acid, polyvinylpyrrolidone, and dodecyl trimethyl ammonium bromide; (2) adding aqueous ammonia solution to the product obtained from the step (1), stirring the mixture to form magnesium salt slurry; and (3) subjecting the product obtained from the step (2) to a reaction at 100 to 250 DEG C for 10 to 36 hours, removing the supernatant after cooling, conducting centrifugal washing on the resulting precipitate, and calcining the precipitate at 250 to 600 DEG C for 2.0 to 6.0 hours under nitrogen conditions, so as to obtain the sandwich-structured nano-adsorbent of carbon-coated magnesium oxide. The adsorbent obtained by the present invention has the advantages of high adsorption capacity and high efficiency, and breaks the industrial application of inorganic nanomaterials in the technical field of water treatment.