Ensiling high-moisture alfalfa with peanut vine not only avoids alfalfa nutrient loss during the wilting stage but also maximizes the use of agricultural waste peanut vine. The appropriate mixed ratio of high-moisture alfalfa and peanut vine has been studied in our previous study. However, the effect of additives on improving the nutrition and fermentation quality of the mixed silage of alfalfa and peanut vine has not been investigated. This study aimed to assess the adaptation and association of Lactiplantibacillus plantarum, cellulase and tannin in the mixed silage of alfalfa and peanut vine alone or in combination on fermentation quality, chemical composition, and microbial communities. The harvested fresh alfalfa and dry peanut vine were cut into 2 cm lengths by a crop chopper and they were thoroughly mixed at a ratio of 7:3. The mixtures were treated with no addition (CK), L. plantarum (Lp, 1 × 10⁶ CFU/g fresh weight), cellulase (Ce, 5 g/kg fresh weight), tannin (Ta, 40 g/kg dry matter), and their combinations (LpCe, LpTa, CeTa, LpCeTa). After 45 days of fermentation, silage treated with Lp, Ce, and Ta had lower pH and ammonia-N (NH₃-N) content and higher concentrations of lactic acid compared with the CK group. LpCeTa-treated silage inhibited protein degradation by reducing pH value and ammonia-N concentrations during ensiling processes. The LpCeTa group increased (p < 0.05) water-soluble carbohydrate (WSC) content and reduced (p < 0.05) acid detergent fiber and neutral detergent fiber contents in mixed silage. Furthermore, the LpCeTa group increased the relative abundance of Lactobacillus and decreased the relative abundance of Enterococcus and Weissella as compared with the CK group. Results of the current study indicated that the combined use of L. plantarum, cellulase, and tannin could serve as a promising strategy for the preservation of ensiling fresh alfalfa mixed with peanut vine and provide a reference for the re-utilization of by-product.