The invention discloses a method for preparing large-particle foam carbon by using a corncob. The method specifically comprises the following steps: drying the corncob, crushing into corncob powder, and then mixing with a catalyst to react for 30-40 minutes; adding an adhesive to react for 60 minutes, then cooling and dehydrating to obtain biomass base expandable slurry; adding a surfactant, a foaming agent and a curing agent to mix and then pouring in a mold, foaming and curing for 15-30 minutes, and drying and curing for 20-30 minutes to obtain charring precursor; then charring at the temperature of 150-350 DEG C for 20-60 minutes, curing for 4 hours at 500-600 DEG C, washing an activation product to be neutral, and drying to obtain the large-particle foam carbon. The agricultural and forestry biomass material corncob is used as a foam carbon matrix, so the consumption of a petroleum base chemical product is reduced, and high value of low-value agricultural waste is realized; the prepared foam carbon has high aperture ratio, large specific surface and strong adsorption capacity and can be applied to the field of environmental engineering such as purification of polluted water body in place of conventional active carbon.