To study the characteristics of the composition of soil nutrients and bacterial communities under different grass-crop rotation modes, this paper measured the content of soil organic matter, total nitrogen(TN), total phosphorus(TP), total potassium(TK), alkali-hydrolyzable nitrogen(AN), rapidly available phosphorus(AP) and rapidly available potassium(AP) with the 5-year alfalfa-1-year wheat(A5W1), 5-year alfalfa-1-year corn(A5C1), 5-year alfalfa-2-year wheat(A5W2) and 5-year alfalfa -2-year corn(A5C2) rotation modes as the subjects of study. On such a basis, this paper further studied the characteristics of the composition of bacterial communities in topsoil under 4 rotation modes in view of 16S rRNA amplicon sequencing. The results indicated that:the content of soil nutrients in the second year of rotation was significantly lower than that in the first year. The study on bacterial composition revealed that the dominant bacterial phyla under the 4 rotation modes were Proteobacteria, Actinobacteria, and Firmicutes. Main advantageous bacterial genera in soil were Propionibacterium, Bacillus, Streptococcus, and Neisseria. In terms of bacterial density in soil, the A5C2 mode was the highest and the A5C1 mode the lowest. According to cluster analysis, the compositions of A5W1 and A5C1 were the closest, followed by A5C2; the composition of A5W2, however, showed a large difference from the former two. The content of soil nutrient was significantly correlated to the advantageous genera(P<0.05 or P<0.01). The results revealed the mechanism behind the improvement of soil fertility and the control of certain soil-borne plant disease microorganisms in the Hexi Corridor Irrigation Area by shifting to wheat and corn after years of planting alfalfa.