Abstract: The dissipation of Saijunmao(chemical name: 2-benzoyl-hydrazono-l,3-dithiolane) and its influence on bacteria populations in paddy soils were studied under a mimicking microcosm soil ecosystem. The results showed that microbial factor in soil played an important role in dissipation of Saijunmao. Due to microbial dissipation, the half-lives of Saijunmao in flooded and non-flooded soil with an original level of 0.2 mg/kg were 2.6 and 5.5 days, and half-lives of 4.0 and 7.3 days for 100 mg/kg level, respectively, which were significantly lower than that in sterilized soil (7.5 and 11.0 days). In contrast to bacteria population in the control, Saijunmao at 0.2 mg/kg exhibited temporal effect on soil bacteria populations and bacteria number recovered to a normal level 7 days after treatment. However, under treatment of Saijunmao at 100 mg/kg, the growth of soil bacteria population was inhibited significantly, and the inhibitory effect could maintained for 28 days.