Achnatherum splendens (Trinius) Nevskia dominates areas of degraded grasslands in China. However, the spatio-chronological patterns of A. splendens and its soil properties remain unknown. We conducted a field study comprising four tussock diameter classes (i.e., an age series) of A. splendens in three degraded grasslands of China (HB: Hebei Province; IM: Inner Mongolia, and QH: Qinghai Province) to determine the soil bacterial community and diversity in the root-associated soil of A. splendens by high-throughput sequencing technology. Achnatherum splendens exhibited strong adaptability and could survive under a low soil water content in the region of IM and in highly alkaline soil in the region of HB. Our findings demonstrated that the biomass of the degraded A. splendens grasslands had a significantly different spatio-chronological distribution, and the plant biomass decreased significantly as the diameter increased. The main bacteria in the root-associated soil of A. splendens showed a significant difference at the spatial scale. Based on the Calinski–Harabasz index, HB belonged to the Actinobacteria enterotype, and IM and QH belonged to the Proteobacteria enterotype, implying that the bacterial structure in root-associated soil of A. splendens had significant spatial differences that were mainly affected by climate factors. The relative abundances of Firmicutes and Gemmatimonadetes decreased significantly as the plant tussock diameter increased. The plant C/N ratio, plant biomass, and soil pH had a positive impact on bacterialβ-diversity between the three habitats, but negatively influenced the bacterial Shannon diversity (α-diversity) in the A. splendens root-associated soil. Our findings highlight that A. splendens not only reduces plant productivity as the diameter increased, but also changes the soil microbial characteristics. Therefore, the spatio-chronological preventive treatment of A. splendens, especially for the older age classes, is very important for improving the productivity of degraded grasslands in China.