Abstract: Abstract: In order to study and develop the effect of biomass C on soil improvement and quality of tobacco, the soil microbial biomass C, N and carbon-to-nitrogen ration were measured, Dark brown soil as the research object, in order to research the soil microbial C, soil microbial N and carbon-to-nitrogen in three key periods of tobacco applied the biomass C in Mudanjiang area. The results showed that compared with control group, the three key periods of the biomass C and N fertilizer adhibition significantly increased the soil microbial biomass C. The biggest increased 63.25%, 58.00% and 69.81%.The biggest soil microbial biomass N content of three periods was 2.29 times, 2.79 times, 2.01 times as much as control groups differentiate. But the carbon-to-nitrogen of soil microbial biomass C had different degrees lower, the reduction of three periods were 24.06%-28.90%, 29.37%-43.47%, 20.64%-34.44% respectively, and the 1200 kg/hm2 biomass C and 5.5kg/hm2 N fertilizer combined application had obvious reduction effect to the carbon-to-nitrogen of soil. Therefore, through the biomass C, N fertilizer could effectively improve the soil microbial biomass C and soil microbial biomass N content and N biological activity of dark brown soil.