The invention discloses a preparation method and application of cow dung biological carbon. The method comprises the following steps: naturally air-drying, crushing and sieving with a sieve of more than 40 meshes cow dung; performing pyrolysis under an air-free condition; mixing a product obtained by pyrolysis and a strong alkaline solution, adding water for rapid and uniform stirring, performing clarifying and filtering, and extracting filter residues; performing washing and drying to obtain activated cow dung biological carbon. According to the preparation method and the application, adsorption and removal effects of the cow dung biological carbon on aqueous solutions of heavy metals lead, cadmium, zinc, copper, nickel and the like are greatly improved, better fixing effects on lead, cadmium, zinc, copper, nickel and the like in soil are achieved, acid soil can be improved, organic matters of the soil are improved, and soil carbon fixation is facilitated. The preparation method is simple, and has far-reaching social significance and economic value, conditions are mild, industrial production is facilitated, additional environmental hazards can be avoided, the utilization efficiency of biomass resources is improved, and the problem of difficulty in degradation and reutilization of agricultural and forestry biomass residues is solved.