Abstract: Sweet sorghum is a variety of promising energy crops with high sugar content, rapid growth rate, and strong resistance to saline-alkaline soils. Appropriate storage is highly required to utilize this feedstock for continuous bioenergy production, particularly harvested in a seasonal manner in China. In this study, the addition of rumen fluid was introduced to sweet sorghum for better ensiling quality during long-term (30 to 60 days) storage. A systematic investigation was made on the ensiling profile, including the organic components, fermentation quality, and enzymatic hydrolysis of sweet sorghum silage. Furthermore, an experimental analysis was performed on the dynamic process of microbial community during ensiling. The sweet sorghum was ensiled at 5 different dosages of rumen fluid: CK, silage without the addition of additive; R1, silage with rumen fluid at 1 mL/100 g, R3 at 3 mL/100 g, R5, 5 mL/100 g, R7 at 7 mL/100 g substrate. The results showed that the contents of dry matter (DM), water-soluble carbohydrates (WSC), crude protein (CP), and ammonia nitrogen (AN) decreased significantly, as both ensiling time and dosage increased in four treatments with the addition of rumen fluid, compared with the CK silages without rumen fluid. The increased rumen fluid and prolonged ensiling contributed to reducing the content of acid detergent fiber (ADF), neutral detergent fiber (NDF), cellulose, and hemicellulose, while elevating the abundance of beneficial Lactobacillus in silages. Lignin in silages at ensiled for 60 days was effectively removed when the maximum dosage of rumen fluid was up to 7 g/100 g. There was a significant decrease in the contents of lactic acid and acetic acid, as well as the pH values in five treatments (CK, R1, R3, R5, R7), with the extension of the ensiling period (P<0.05). The contents of lactic acid and acetic acid in R7 and R5 were higher than those of treatments after 60 days, which were 9.42 g/kg (R7) and 29.08 g/kg (R5), respectively. The dominant phylum in CK silages was still Proteobacteria after ensiling, where the relative abundance was up to 96.1% and 81.2% after 30 and 60 days, respectively. However, the bacterial communities changed significantly, due to the fact the bioaugmentation of rumen fluid resulted in the coexistence of Proteobacteria and Firmicutes in bioaugmented silages. After bioaugmented ensiling for 30 days, Lactobacillus and Acetobacter became enriched in the range of 18.2-46.2% and 3.61-18.0%, respectively, as the dosage of rumen fluid increased. After 60 days of ensiling, the relative abundance of Lactobacillus increased from 45.2 (R1) to 81.1% (R7) with the increase of rumen fluid dosage. The addition of rumen fluid also enhanced the reducing sugar yield during enzymatic saccharification for 72 h. Specifically, the yields of reducing sugars in all treatments at 30 days significantly increased by 26% (CK), 30% (R1), 34% (R3), 40% (R5), and 44% (R7), respectively, compared with the raw sweet sorghum. The bioaugmented ensiling improved substantially the yield of reducing sugars, compared with the CK silages, indicating a positive correlation to the dosage of rumen fluid. It was also found that the 72 h reducing sugars yields of all silages at 60 days were significantly higher than those at 30 days. The R7 silage achieved the highest reducing sugars yield of 795.4 mg/g after 60 days of silage, indicating a linear correlation to the high biodegradation potential. The addition of rumen fluid can effectively improve the ensiling quality and biodegradability of ensiled sweet sorghum, thereby serving as biological pretreatment via bioaugmentation. This finding can provide the promising ethanol utilization of sweet sorghum.