The invention discloses a porous carbon adsorbent for removing mercury and a preparation method of the porous carbon adsorbent, and belongs to the technical field of smoke mercury control. The porouscarbon adsorbent is high in specific surface area, strong in pore structure and rich in surface activity level, so that the mercury can be efficiently removed. The preparation method mainly comprisesthe following steps: stirring and heating a biomass solution to obtain a biomass gel; performing freeze drying to obtain a carbonizing precursor; carbonizing the precursor under inertia atmosphere; activating the precursor through CO2; and modifying the material through H2S so as to obtain the porous carbon adsorbent. The preparation method is simple, reliable, free from high-corrosion activatingagent, low in cost, and easily industrially. Therefore, the adsorbent is a mercury removing material with the industrial application prospect.