• 10 years of CC was a cut-off point in separating soil bacterial community structures.
• Soil pH and P were well associated with changes of diversity and community structures.
• N fixation bacteria were increased with successive year, but P, K solubilizing bacteria decreased.
• Monocropped alfalfa simplified the complexity of the cooccurrence networks.
Alfalfa is a perennial herbaceous forage legume that is remarkably and negatively affected by monocropping. However, the contribution of the changes in bacterial communities to the soil sickness in alfalfa have not been elucidated. Therefore, we investigated bacterial community structures responses to monocropped alfalfa along the chronosequence. Continuous cropping remarkably reduced bacterial alpha diversity and altered community structures, and soil pH, total P and available P were strongly associated with the changes of bacterial diversity and community structures. Intriguingly, 10-year of monocropped alfalfa might be a demarcation point in separating soil bacterial community structures into two obvious groups that containing soil samples collected in less and more than 10-years. The relative abundances of copiotrophic bacteria of Actinobacteria and Gammaproteobacteria were significantly increased with the extension of continuous cropping years, while the oligotrophic bacteria of Armatimonadetes, Chloroflexi, Firmicutes and Gemmatimonadetes showed the opposite changing patterns. Among those altered phyla, Actinobacteria, Chloroflexi, Alphaproteobacteria and Acidobacteria were the most important bacteria which contributed 50.86% of the community variations. Additionally, the relative abundances of nitrogen fixation bacteria of Bradyrhizobium and Mesorhizobium were obviously increased with successive continuous cropping years, while the abundances of Arthrobacter, Bacillus, Burkholderiaceae and Microbacterium with potential functions of solubilizing phosphorus and potassium were remarkably decreased after long-term continuous cropping. Furthermore, bacterial cooccurrence patterns were significantly influenced by continuous cropping years with long-term monocropped alfalfa simplifying the complexity of the cooccurrence networks. These findings enhanced our understandings and provided references for forecasting how soil bacterial communities responses to monocropped alfalfa.