The invention relates to a method for preparing modified 
biomass charcoal for passivating lead and 
cadmium in soil. The method comprises the following steps: A, 
grinding and screening 
biomass raw materials; B, respectively dissolving FeSO4.7H2O and KMnO4 in water, uniformly mixing, regulating the pH value to 7, thus obtaining iron and 
manganese oxide suspension; C, adding the ground 
biomass raw materials into the iron and 
manganese oxide suspension, uniformly stirring, soaking, 
drying at the temperature of 105 DEG C for 6 hours, thus obtaining a biomass material loaded with iron and 
manganese oxides; and D, adding the biomass material loaded with iron and manganese oxides into a reactor, raising the temperature to 450-650 DEG C under 
oxygen-limited conditions, carrying out a 
pyrolysis carbonization reaction, thus obtaining the modified biomass 
charcoal. According to the method disclosed by the invention, the iron and manganese oxides are distributed in the surface and pores of the biomass 
charcoal, a 
passivation effect of the biomass charcoal on 
heavy metals lead and 
cadmium can be obviously improved, the environmental benefits of the biomass charcoal for treating contaminated soil are effectively improved, and the remediation cost of heavy 
metal contaminated soil also can be effectively reduced.