Biochar has received widespread attention as a promising amendment for heavy metal stabilization due to its abundant porosity and surface functional groups. However, the role of biochar-derived dissolved organic matter (BDOM) is usually overlooked. In this study, we systematically investigated the leaching dynamics of BDOM from garden wastes through hydrothermal carbonization (HC), pyrolysis (PC) and hydro/pyrolysis (HPC) and explored their impacts on Cr(VI) environmental behavior in an extremely acidic environment. Results showed that BDOM leaching dynamics followed the first order model, and HC leached more BDOM than PC and HPC, especially for Ar-P, SMP and Ha-L fractions. Although carbonized using various methods, the biochars displayed a similar adsorption capacity for Cr(VI) at an environmental-related concentration of 2 mg L⁻¹. The presence of BDOM accelerated the Cr(VI) adsorption rate on biochars due to their pre-complexing. Simultaneously, HC-BDOM acted as an electron donor and participate in Cr(VI) reduction directly, resulting in the synchronous reduction of Cr(VI) during its adsorption process. However, PC- and HPC-BDOM preferentially acted as electron acceptors, thus competing with Cr(VI) for Fe(II) oxidation and decreased the Cr(VI) reduction rate. This study suggested that biochar from garden wastes has a great remediation potential for Cr-contaminated land and that BDOM (especially HC-BDOM) plays a significant role in increasing soil organic matter, stabilizing heavy metals and detoxifying toxic substances by oxidation-reduction.