A method of preparing a bio-adsorbent relates to the bio-adsorbent and comprises the steps: cleaning, drying, grinding and filtering raw biomass materials to obtain biomass powder; mixing citric acid and the biomass powder, soaking, heating and stirring to obtain solid-liquid mixture; performing suction filtration to the obtained solid-liquid mixture and collecting filtrate; mixing the filtrate and chitosan, heating, stirring, adjusting the pH value to be 8-10, separating solids, continuously stirring, cooling, performing suction filtration, washing to be neutral and obtaining neutral filter residues; drying and grinding the neutral filter residues to obtain the bio-adsorbent. The bio-adsorbent can be applied to the preparation of the bio-adsorbent for heavy metals and dyeing waste water. The preparation method is simple in operation and wide in applicable range; and the waste acid treatment liquid of various agricultural and forestry residue biomasses can be used for preparing the low-price adsorbent available for heavy metals and dyeing waste water by utilizing the preparation method provided by the invention.