Abstract: The effects of chemical mutation by hydroxylamine hydrochloride on the activity and growth of strains and the tailing leaching system were investigated with the superior bacteria of Acidthiobacillus ferrooxidans (T.f₆) as an original strain. The results showed that chemical mutation by hydroxylamine hydrochloride produced an evident mutation in the strain T.f₆, which can improve its bioactivity and bioleaching properties. The strain T.f₆ had the best oxidation after chemical mutation by hydroxylamine hydrochloride with a mass fraction of 1.0%. The oxidation rate of Fe²⁺ reached 100% after culturing for 32h, which was 32h in advance in comparison with the original strain. After a 30d leaching, the copper leaching rate of the mutant strain T.f₆ was improved by 20.7% as compared with that of the original strain and it was 85% higher than the acid leaching rate. In addition, utilizing the mutant strain could shorten the leaching time of 5 to 8d. It is indicated that the effect of bioleaching with the mutant strain is better than that with the original strain, and much better than that of the acid leaching. Scanning electron microscope analyses of the original strain and the mutant strain showed that the shape of the mutant strain did not change but the cell size did, the surface changed smooth and extracellular secretion occurred, while the clustering of cells was obvious.