The surface of wheat straw-based biochar was modified by a pretreatment using H3PO4. Biochars with different surface characteristics were obtained and characterized. Their reducing and sorption capacities for aqueous Cr(VI) were evaluated. The removal of Cr(VI) were enhanced with an increase in biochar surface acidity. The sorption performance was related to the equilibrium concentration. Oxygen-containing groups were involved in Cr(III) complexation, some of which were formed due to Cr(VI) oxidation. The competitive sorption of proton was evident for over protonated biochars at low range. Biochar porosity also played a role. The influences of other factors were also discussed.