The invention relates to a preparation method of ZnCl2-microwave activated corncob activated carbon for treating basic dye wastewater. The preparation method comprises the following steps: step 1, pretreating raw materials; step 2, weighing the raw materials; step 3, performing heating reflux; step 4, performing microwave activation; step 5, washing and filtering; step 6, drying; and step 7, cooling and storing. The prepared activated carbon has a large number of micropore structures, the pore size distribution is uniform, the surface contains rich oxygen-containing functional groups, dye molecules can be attached and deposited on the activated carbon adsorbent easily, and the activated carbon is more suitable for adsorption. The removal rate of methylene blue dye can reach 99.9% at most,the maximum adsorption capacity can reach 440.5 mg / g, the product can be used for treating wastewater containing basic dye, and the adsorbent is simple in preparation process, low in cost, environmentally friendly and easy to popularize.