Abstract: The effects of paraquat on the population amount of soil microflora and their activity in soil at three different densities were studied in this experiment. The results indicated that paraquat had a significant effect on soil microflora in thirty days. Paraquat of 0.05~5mg/kg inhibited the growth of funguses, but stimulated the growth of the population of cellulose utilizing microorganism and nitrifying bacteria. The low-density paraquat stimulated the growth of bacteria and actinomyces, but high-density paraquat inhibited their growth. The results also showed that soil nitrification and soil cellulose decomposition was stimulated. And the effect of stimulating or inhibiting had recovered normal level with prolonging of time of adding pesticide.