The invention discloses a preparation method of a heavy metal ion adsorbent. The heavy metal ion adsorbent is mainly prepared by the following steps: in-situ aqueous phase heating and bonding of an extract in water-based ink printing wastewater, thermal insulation and pore-forming, dehydration, drying, and one-step carbonization activation. The preparation method specifically comprises the following steps: adding a cation exchange material into the high-concentration water-based ink printing wastewater, stirring and reacting, and controlling the reaction until pH of the wastewater is 3-6; heating and stirring to make the fine particles and the cation exchange material adhere to each other to form an internally porous large granular extract; carrying out thermal insulation for full bondingand deformation of the extract and shrinkage of volume and dehydration, and forming pores by a water phase; discharging water when the volume of the extract is fully shrunk, adding a saturated potassium hydroxide solution and cooling; taking out the extract, and drying at low temperature; crushing the dried extract and carrying out one-step carbonization activation under the protection of nitrogenat 600-800 DEG C for 1-4 h so as to prepare the heavy metal ion adsorbent.