Dissolved organic carbons in biochar (BDOM), obtained from thermal treatment (i.e., pyrolysis) of biomass, is of great importance due to their excellent redox properties and capacity to remove contaminants from the aqueous and soil environment. However, little is known about the intimate relationship between redox activity of BDOM and pyrolysis temperatures of the biomass. In this study, BDOMs were extracted from wheat straw biochar at different temperatures (from 300 °C to 700 °C). The physicochemical analyses indicated that the quinone and aromatic moieties in BDOM increased with the increase in pyrolysis temperature up to 500 °C, and then decreased as the temperature continued to rise. The results of electrochemical analysis revealed that the electron transfer capability (ETC) reached a maximum for the BDOM-500 with an electron donor capability (EDC) of 0.14 mmol (g C) and electron accepting capability (EAC) of 0.31 mmol (g C), which were both significantly higher than that of other as-prepared BDOMs. The EAC and EDC of BDOM samples both followed the order BDOM-500 > BDOM-400 > BDOM-600 > BDOM-300 > BDOM-700, demonstrating that the quinone and aromatic groups highly contributed to the redox activity of the BDOM. For Cr(VI) removal, the highest removal efficiency (~77%) was achieved in the presence of BDOM-500 and lactate as an electron donor, which was consistent with the profiles of ETC. These findings present a guidance for the optimization of BDOM that efficiently mediates pollutants removal for environmental remediation.