Rice-straw is rich with nutrient elements for crop growth and it is an excellent natural resource for humans. Most of rice-straw is incinerated in farms of China; a lot of CO2 is generated and released to the environment which may cause “greenhouse effect". To avoid this kind of problem and utilize rice-straw as a nutrient resource, rice-straw returned to the field right after the harvest has been becoming a common way in rice production in southern area of China. Many related field studies showed that the soil quality could be improved by rice-straw returned to the field and the crop growth could be promoted which resulted in higher yield. To investigate the effect of returning quantity of rice-straw to the field on biological characteristics of soil and provide scientific rational to the farmers for applying suitable amount of rice-straw, a field experiment with the different returning amount of 0%, 33%, 67%, 100% of harvested rice-straw to the field was conducted and the effects on soil microbial flora and activity in paddy soil were determined. The populations of aerobic bacteria, anaerobic bacteria, actinomyces and fungi, microbial activities in soil showed the highest at late-rice maximum tillering stage. Those populations and the microbial activities in soil applied with rice-straw were more than that in soil without the application of rice-straw. The order of aerobic bacteria amount in soil applied with different amount of rice-straw was 0%>33%>67%>100% at late-rice maximum tillering stage. And the number of aerobic bacteria in soil without rice-straw application was the lowest. The order of anaerobic bacteria amount in soil was 33%>67% or 100%>0% of rice-straw. The situation of fungi in soil was same as aerobic bacteria. The number of actinomyces in soil covered with rice-straw was more than that without rice-straw. Soil microbial activity was stimulated by returning of rice-straw, the order of soil activity was 67%>100%>33%>0 of rice-straw. The activities of ammonification and nitrification at late stage showed an ideal situation when 33% of rice-straw was returned. Cellulase activity in soil showed the highest at late-rice maximum tillering stage, and xylanase activity in soil showed the highest at late-rice maximum booting stage. It was also demonstrated that applying 25005000 kg/hm2 rice-straw to the field was suitable under most of the conditions.