Abstract: The effects of a magnetic field on the pH and COD_Cr mass concentration inthe drain water and on the mass concentration of H₂S, NH₃, H₂, CH₄, CO₂ and VOCs in the biogas from a dominant bacterium Cr(Ⅵ) removal biosystem were investigated. The results showed that the final pH in system B with a magnetic field was slightly lower than that in system A without a magnetic field, whereas the COD_Cr removal rate of system B was significantly higher than that of system A. The introduction of a magnetic field was found to restrain the production of H₂S and NH3. It was observed that, with an increase of the original Cr(Ⅵ) mass concentration from 30 mg/L to 60 mg/L, the outputs of H₂S and NH₃ from both systems increased by 28.67%-32.87% and 62.65%-70.89%, respectively. When the original Cr(Ⅵ) mass concentration was 30 mg/L, the mass concentrations of CH₄ and CO₂ in thebiogas from system B were 210% and 33.33% higher, respectively, than those from system A,while the VOCs mass concentrations of system B were 14.75% lower than that of system A. When the original Cr(Ⅵ) mass concentration was increased to 60 mg/L, it was found that in both systems the CH₄ mass concentrations in the biogas were equal, andthe CO₂ mass concentrations obviously decreased while the VOCs mass concentrationsincreased.Additionally, H₂ was not detected in either system during the test period. The mechanism by which the magnetic field affects anaerobic biogas production was illustrated in this paper from the viewpoint of the anaerobic physiological-biochemical process.