Effects of iodosulfuron-methyl-sodium on catalase in soil and soil respiration were studied by simulation in laboratory. The results showed that when the concentration of iodosulfuron-methyl-sodium was 7 times as high as is recommended for field application also had no significant effect on soil catalase activity; But when the concentration was 1mg/kg, the catalase activity of soil was slightly inhibited at first, then stimulated, and at last restored; In the different soils, the higher the organic matter and the lower the pH, the greater the effect of the substance on catalase activity. Iodosulfuron-methyl-sodium also had some effects on soil respiration. The higher the concentration of the substance, the greater the influence in the early days. But as time went by, inhibition was gradually replaced by stimulation. After 12 days respiration of the soil treated with iodosulfuron-methyl-sodium was similar to that in CK. Results of the experiments showed that iodosulfuron-methyl-sodium did little harm to soil microorganisms, and it was a kind of herbicide of low toxicity or no-real harm.