Abstract: Four kinds of biochars from rice stalk, corn stalk, banana peel and pomelo peel were prepared by oxygen cracking method at a high temperature of 500 ℃. The carbon yields of biochars were calculated and the scanning electron microscope (SEM) , fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) were used to characterize the structures, morphologies, surface functional groups, and crystallinity of biochars, and their relationship with adsorption of biochars were analyzed. The results showed that the carbon yield of banana peel was the highest (39.5%) , and that of pomelo peel is the lowest (32.8%) . Bore aperture of straw biochar was larger than that of pericarp biochar, which was due to the higher content of cellulose and hemicellulose in straw which would decompose at high temperatures. Straw biochar had a higher degree of crystallization, better stability, and better treatment of pollutants. Near 1 600 cm^-1, functional groups such as carboxyl group, ester group or aldehyde group of straw biochar had greater strength than those of pericarp biochar, which would promote the adsorption of methylene blue. The adsorption abilities of the four type of biochar to methylene blue from high to low were rice stalk biochar, corn stalk biochar, banana peel biochar and pomelo peel biochar, indicating that straw biochar could remove methyl blue from dye wastewater more effectively.