By taking acidophilic bioleaching bacteria (Acidthiobacillus ferrooxidans,At. f for short) as the research objective,this paper discusses the effect of Mg²⁺ on the oxidation activity of At. f and analyzes the corresponding in- ternal mechanism by means of Zeta potential and infrared spectroscopy.The results indicate that,when the mass concentration of Mg²⁺ is less than 5. 0g/L,after a cultivation for 35h,the pH value of the system decreases from 2. 00 to 1. 70,the oxidation reduction potential increases from 390mV to more than 600mV,the Fe²⁺ oxidation rate is up to 100%,and the bacteria are in an exuberant growth period; and that,when the mass concentration of Mg²⁺ is more than 5. 0g/L,the reduction rate of pH value,the rise rate of the oxidation reduction potential,the oxida- tion rate of Fe²⁺ and the bacteria growth rate all decrease,and the decreasing rate is positive to the mass concentra- tion of Mg²⁺ .Zeta potential tests show that the bacteria isoelectric point is 3. 6 under the action of 20. 0g/L magne- sium ions,which is higher than that of the bacteria cultivated in 9K medium,namely 2. 7.This is the macroscopic cause of the structure variation of bacterial cell walls.Moreover,IR results show that,under the action of magne- sium ions,the functional groups of the bacterial cell walls crosslink,which results in the disappearance of N—H absorption peak as well as the migration of —OH，—C＝O，—CH₃ ,—CH₂ ,—P＝O and —CN.Thus,the structure of the cell surface protein changes and the oxidation activity of bacteria decreases.