The effects of brine addition on the dynamic changes in nitrogen transformation, and bacterial community of fermented Allium chinense (A. chinense) bulbs, as well as screening and identification of lactic acid bacteria (LAB) with nitrite degradation function were explored in this work. In contrast with natural fermentation (NC) group, supplementation with aged pickle brine (PBa) declined the pH value. In terms of nitrogen metabolism, PBa fermentation could enhance the degradation of nitrite via activating nitrite reductase (NIR). Besides, the ammonium contents showed a small fluctuation without significant difference during fermentation. Furthermore, PBa could alter the microbial diversity of A. chinense bulbs to modulate nitrogen metabolism-related genes including nirA/B/D and narK/G/H. Correlation analysis confirmed that the LAB abundances exhibited significant negative relationships with the levels of nitrite. These results inferred that the microbial pathways of nitrite degradation in A. chinense bulbs might be attributed to denitrification. Accordingly, the present findings suggested that brine fermentation could promote the nitrogen metabolism in A. chinense bulbs via regulating the microbial community and expression of denitrification-related genes, thus shortening the fermentation time compared to the NC group. Moreover, one strain of LAB (L7) was isolated from PBa group by pure culture method, which showed high similarity to Lactiplantibacillus argentoratensis HBUAS570840, and strong nitrite-lowering ability, high salt tolerance, and strong antibacterial activity. Therefore, the addition of brine can be utilized as an effective strategy to modulate the nitrogen metabolism of A. chinense bulbs.