Biochar is a solid product with high aromatization, high carbon and high stability produced by special treatment of raw materials under partial hypoxia or complete deoxygenation. Because the performance of biochar was prepared from different parts of the same plant often different, in this paper, hemp was taken as the research target, KOH and H3PO4 were used as activators, to activate the different parts (stem, skin and root) of hemp. Biochar materials from different sources were prepared by hydrothermal carbonization method, activation method and carbonization-activation method. The as-prepared products were used to adsorb rhodamine B (RhB) and methylene blue (MB) solutions, and their adsorption activities were evaluated. XRD, SEM, TG-DSC, N2-BET, UV-Vis and other characterization methods were used to analyze the properties of the products in order to explore the effects of carbonization temperature, activation temperature and activator type on the properties of biochar. Using flax rod as raw material, phosphoric acid as activator, carbonization temperature 200 ℃, activation temperature 820 ℃, the biochar pores were prepared with uniform distribution, a large number of cross sections in tubular shape. The specific surface area of the biochar is as high as 1 247.63 m2/g. It is also displayed great adsorption capacity for RhB and MB. After nearly 1 h, the 10 mg/L RhB solution has all faded, and the adsorption rate is as high as 100%.