The invention relates to a method for preparing a spherical carbon heavy metal adsorbent. The method is characterized by taking pentosan extracted by a bleached hardwood pulp alkaline method as a raw material and pure water as a solvent, and comprising the steps of after treatment at high temperature and high pressure under a hydrothermal condition, centrifugally separating so as to obtain a dark brown solid product, namely carbon spheres rich in acid oxygen-containing functional groups, washing the dark brown solid product for many times by distilled water and absolute ethyl alcohol till filtrate is clear, and drying in vacuum so as to obtain the spherical carbon heavy metal adsorbent. The method has the main characteristics that the pentosan extracted from a byproduct of pulping and papermaking is taken as the raw material and is environmentally friendly, cheap and easily available, water is taken as the solvent, the reaction needs no modifier and further oxidation, the adsorbent containing a large quantity of hydroxyl and carboxyl oxygen-containing functional groups is formed through dehydration, condensation, aromatization and carbonization, the adsorbent and metal ions can have chelation, and the adsorbent can be used as high performance adsorbent for adsorbing heavy metal ions like Pb(II) and Cd(II).The hydrothermal reaction adopted in the method has a simple operation process and the adsorbent is low in cost and has an excellent adsorbing effect.