Abstract: The aim is to find the most suitable condition of straw biomass carbon returning to field in Nantong. Seven treatments were set up in experimental station of Nantong Vocational College of Science and Technology, including no biomass carbon (CK), rice straw carbon (3SD), rice straw carbon (4SD), rice straw carbon (5SD), corn straw carbon (3YM), corn straw carbon (4YM), and corn straw carbon (5YM). The soil physical properties, nutrient content, enzyme activity and microbial community structure in paddy field were determined after applying biomass carbon. The effects of different char- making conditions and biomass char application amounts on growth and development of rice were evaluated by redundancy analysis of rice yield and multiple indexes. The results showed that both 5SD and 5YM treatments could increase rice yield by 28.01% and 29.86%, respectively. 5SD treatment had a better yield increase effect on rice. Compared with CK at mature stage, 5SD treatment increased rice yield by 65.99% , and increased soil microbial biomass by 243.90% . However, the decrease range of soil microbial biomass carbon, microbial biomass nitrogen, soil enzyme urea activity, and soil invertase activity in paddy soil was - 41.01% , - 82.78% , - 48.92% and - 57.29% , respectively. In this experiment condition, biochar made from rice straw at 500℃ had the best effect on rice yield increase, and could also improve soil physical and chemical properties and soil enzyme activities.