We chose wetland soils in inner-dams, between-dams, outer dam edges, and control (soils unaffected by drainage) to investigate the effects of oilfield discharged water on soil microbial biomass in constructed wetland in arid Gobi region. The phospholipid fatty acid (PLFA) analysis was used to study the effects of oilfield discharged water on biomass of soil bacteria, fungi and actinomycetes. The results show that pH and soil bulk density had no significant differences among the soils from inner-dams, between-dams, outer-dam-edges, and the control. Soil water content, electrical conductivity, total soluble salt content, and total carbon content were all highest in the between-dam soils and were remarkably higher than those in the inner-dam soils. Except water content, the physical properties and nutrient contents had no significant differences between the control and other three soils. Soil bacteria, fungi and total microbial biomass increased from the inner-dams to outer dam edges. The soil total PLFA, bacteria, fungi and actinomycete biomass were positively correlated with the soil total nitrogen content. The soil arbuscular mycorrhizal fungi biomass was positively correlated with the soil total carbon content. The fungi and arbuscular mycorrhizal fungi biomass were positively correlated with the total petroleum hydrocarbon content. The study showed that oilfield discharged water increased soil microbial biomass in the outer dam edges.