[Objective] The effects of different application amounts of super-absorbent polymer(SAP) on soil physical characteristics and microbial activity under growing poplar were researched to provide theoretical basis and technical reference for high-yield and water-saving practice modes in Chinese poplar plantation.[Methods] Four treatments, including CK(conventional irrigation), CL(60% conventional irrigation quantity co-applied with 10 g SAP), CM(60% conventional irrigation quantity co-applied with 30 g SAP) and CH(60% conventional irrigation quantity co-applied with 50 g SAP) were set. Their effects on soil physical properties, water stability of crumb structure, microbial populations, microbial respiration and metabolic quotient under poplar seedlings grew in pots were determined.[Results] The CM treatment significantly reduced soil bulk density and obviously increased non-capillary porosity. The soil bulk densities decreased in 13.71%, 5.31% and 7.76% as compared with those of CK, CL and CH, respectively. The formation of soil large aggregate(>0.25 mm size) was also promoted by the CM treatment. Moreover, the CM treatment observably increased total microbial populations and the contents of microbial biomass carbon and microbial biomass nitrogen. And CM treatment significantly enhanced microbial respiration, having 30.77%, 11.84% and 7.59% increases in microbial respiration rate in comparison with the ones of CK, CL and CH, respectively. In addition, the metabolic quotient was reduced by the CM treatment, having 4.81%, 8.29% and 9.74% decreases as compared with the ones of CK, CL and CH, respectively.[Conclusion] As a result, the proper water-saving irrigation measure(60% conventional irrigation quantity co-applied with 30 g SAP) is beneficial to improve soil physical environment and enhance soil microbial activity under poplar seedlings.