Abstract: This thesis discusses the content and influential factors of．soil microbial biomass nitrogen．deals with the biologic validity of,soil microbial biomass nitrogen and the factors that influenced biologic fixation of soil inorganic nitrogen，expands the determination method of soil microbial biomass nitrogen and also makes clear the importance of microbial biomass nitrogen in the process of cycle and transformation of soil nitrogen．The soil microbial biomass nitrogen is an important transformation link of soil nitrogen．It is also the body of soil available nitrogen repertory It plays an important regulation role to nitrogen supply of crops．The biologic fixation of soil inorganic nitrogen also plays positive roles in decreasing lose of nitrogen，increasing efficiency of nitrogen fertilizer and protecting environment．