To study the impact of stable weak magnetic field on the Cr(VI) removal efficiency of predominated strains in ASBR system, the choice of the optimum magnetic density and its effect should be considered chiefly. At different magnetic densities, the growth and propagation rates of predominated strains in solid or liquid mediums and their capabilities of removing Cr(VI) were compared. The results showed that the optimum magnetic density was 6.0 mT. To meet the state first-class standard of effluent discharge, it took 2-5 h more in the plant wastewater treatment than in the synthetic wastewater treatment, but the presence of magnetic field made the reaction time up to par to decrease 1 and 2-3 h, respectively, compared with that of the control. The magnetized magnetic powder could improve the sludge sedimentation capability, turbidity of outflow water and efficiency of bio-system.