The objective of this study was to evaluate the potential of tannin acid (TA) derived from Chinese gallnut, with different doses for modulating ensiling characteristics, methane (CH4) emission, ruminal fermentation profile and microbiota of alfalfa silage. Second-cut alfalfa was wilted for 6 h to dry matter (DM) 439 g/kg fresh weight and chopped to 2 cm length. The chopped forage was ensiled with 5 TA levels (0, 10, 20, 40, and 60 g/kg DM), and each treatment was ensiled in triplicate in vacuum-sealed polyethylene plastic bags for 60 d. All TA treatments improved the fermentation quality of alfalfa silage, indicated by lower (P < 0.05) DM losses, pH values, and non-protein nitrogen concentrations. The acetic acid concentration markedly decreased (P < 0.05) with the greater levels of TA, leading to a reduction in the aerobic stability of silage. Ruminal CH4 production decreased (P < 0.05) with increasing TA levels from 20 to 60 g/kg DM, by which the CH4 concentration decreased by 10.3 to 12.4% compared with control. The DM, crude protein and neutral detergent fiber degradation linearly decreased (P < 0.05) with increasing TA levels, while propionic acid molar proportion and microbial protein production linearly increased (P < 0.05) with increasing TA levels, but the greatest level of 60 g/kg DM did not further increase these variables. The abundance of methanogens, anaerobic fungi, Ruminococcus albus, Ruminococcus flavefaciens, and Prevotella ruminicola as well as the activity of cellulase and protease linearly decreased (P < 0.05) with TA inclusion, whereas Fibrobacter succinogenes was enhanced. Spearman correlation analysis showed high correlations between ruminal microbial abundance and CH4 production/fermentation parameters. The results indicated that TA, used as an environmentally friendly modifier prior to ensiling alfalfa can improve silage quality and modulate rumen fermentation pattern. However, TA from Chinese gallnut with levels greater than 40 g/kg DM is not recommended for addition to alfalfa silage.