Abstract: In order to understand the soil micro-ecological effects of different straw returning in tobacco fields, a three-year field trial was conducted to study the effect of different amount of wheat straw and maize straw returning on soil enzyme activities and soil bacterial community diversity. The results indicated that the activities of soil invertase, alkaline phosphatase and urease were all promoted in a certain degree by straw returning. The activities of soil invertase and unease in maize straw returning were higher than those in wheat straw returning at the same amount. The soil bacterial communities were both decreased in low (1500 kg/ha) and middle (4500 kg/ha) amount of maize and wheat straw retuning compared with CK, while they remained unchanged or increased slightly in high (7500 kg/ha) amount of straw returning. The similarity coefficient of soil bacterial communities between the 7500 kg/ha maize straw retuning treatment and other treatments was only 0.57, which indicated that its soil bacterial community structure changed greatly. Furthermore, some specific bands were found in DGGE band spectrums with low, middle and high amount of maize straw returning. They were identified as Lactococcus, Uncultured bacterium and Nitrospira individually by band cloning and sequencing. In total, both of maize and wheat straw returning could improve soil invertase, alkaline phosphatase and urease activities. High amount of maize straw could increase the richness and diversity of soil bacterial communities in tobacco planting fields.