Abstract: In order to better understand the distribution of soil microorganism in the Populus euphratica forests in Xinjiang of northwestern China,this paper studies and compares the population and quantity of bacteria,fungi and actinomycetes in the soil of four kinds of P.euphratica forests,which are young forests,middle-aged forests,over-mature forests and effete forests.The results showed that there were evident differences in the microorganism biomass and composition rates of four growing stages of natural P.euphratica forests in Xinjiang,and the dominant and special microorganism existed in all different layers of soil section.The vertical distribution showed that the microorganism biomass decreased with the increase of soil depth,and the population of microorganism was the least in 31-40 cm depth of soil.The microorganism consisted of bacteria,which were the chief content and distributed widely,and actinomycetes as well as fungi,which were scarce in some layers of soil section.Aspergillus was the dominant genus among the 11 genera of fungi isolated from soil in the different growing stages of P.euphratica forests.