Technical background: Heavy metal wastewater is a difficult problem in industrial sewage treatment in my country and has caused serious environmental pollution. Farmland in many places is irrigated with water with excessive heavy metals, resulting in the produced grain being polluted by heavy metals and becoming inedible. Adsorption is an effective method to treat wastewater containing heavy metal ions. At present, rice husks are used to adsorb and remove copper ions and lead ions in wastewater, but the effect is not very ideal. The removal rate of mercury ions is only about 80%. Moreover, rice husks are easily decomposed by microorganisms in nature, and the adsorbed heavy metal ions will be released into the environment. In addition, there are also methods of adsorbing lead ions by using chitosan oligo/polysaccharide series derivatives. Although the effect of this method is good, the synthesis of this type of adsorbent is relatively complex, the cost is high, and it is not easy to promote and use.  Innovative: The purpose of the present invention is to overcome the shortcomings in the prior art and provide a preparation method of a special adsorbent for heavy metal ions with simple process, low cost and good effect. Project description: The preparation method of the heavy metal ion adsorbent of the present invention comprises: adding low-grade coal crushed to less than 180 meshes into urban sludge, and the dry basis mass ratio of the low-grade coal to urban sludge is: 1:9 - 5:5; and then at a temperature of 560 DEG C.~Heating and carbonization in a carbonization furnace at 900℃ for 20~40 minutes to obtain a carbonized solid product, and when the temperature of the carbonized solid product drops to room temperature, it is crushed to obtain the heavy metal ion adsorbent. The particle size of the crushing is below 100 meshes.  Prospects: The invention uses dehydrated sludge from an urban sewage treatment plant as the main raw material, adds finely ground low-grade coal, uniformly mixed, carbonized at a high temperature, and then ground to obtain the sludge carbonaceous adsorbent. The adsorbent can be applied to the adsorption treatment of lead-containing simulated wastewater: 50ml of simulated wastewater with an initial concentration of Pb2 + of 35mg/L was placed in a 250-ml conical flask with a stopper. Under the conditions of oscillation frequency of 150r/min, adsorption time of 1 hour, temperature of 303K, pH=7, and adsorbent dosage of 0.15 g, the removal rate of Pb2 + can reach 94.98%. The adsorbent has simple manufacturing process, low cost, good use effect, easy promotion and use, and wide practicality.