The invention discloses biomass carbon and a preparation method and application thereof. The preparation method comprises the following steps that (1) biomass raw materials are preliminarily carbonized, so that preliminarily carbonized biomass is obtained; and (2) the preliminarily carbonized biomass obtained in the step (1) is deep carbonized, so that the biomass carbon is obtained. Preliminary carbonization treatment in step (1) is preliminary pyrolysis, and the temperature of preliminary pyrolysis is 100-300 DEG C; and deep carbonization treatment in the step (2) is deep pyrolysis, and thetemperature of deep pyrolysis is 350-600 DEG C. The technological process is clear in structure, techniques are mature and operable, the characteristics of small investment, quick effect and high output are achieved, the obtained biomass carbon can effectively improve physical and chemical properties of soil, maintain water, retain nutrients in the soil and promote growth of beneficial microorganisms in the soil, the crop yield and quality improvement are comprehensively promoted, and the plant economy income of farmers is increased.