Abstract: The effects of paraquat at five different concentrations on the population of microorganism and the growth of bacteria, actinomyces and fungi in soil were studied. The results show that the herbicide inhabits the population of microorganism and the growth of bacteria, actinomyces and fungi in soil. And the inhibiting effect is more obvious with the increasing of concentration of the herbicide. The inhibiting rate of the population of bacteria is 13.3%～100% and the diameter of inhibiting circle is 0.07～0.93cm, that of the population of actinomyces is 8.6%～100% and that of inhibiting circle is 0～3.00cm, that of the population of fungi is 7.6%～100% and the inhib iting rate of the growth of fungi is 1.4%～77.3%. The inhibiting effect becomes weak with the prolonging of time.