Abstract: Compared to unfermented water extract (WE) of Pericarpium Citri Reticulatae (PCR), the effects of Bacillus sp. fermentation on the main components and in vitro antioxidant activity of water extract (FWE) of PCR and its prolongation of the lifespan of Caenorhabditis elegans (C. elegans) were investigated. The total flavonoids, total polyphenols and total sugars of the two water extracts were determined. The in vitro antioxidant activities of two water extracts were compared using four indicators: 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging rate, hydroxyl radical scavenging rate, ferrous ion chelating rate and total reducing capacity. The in vivo antioxidant activity of the two water extracts was evaluated by feeding the C. elegans at low (1 mg/mL), medium (3 mg/mL) and high (5 mg/mL) doses. The results showed that the total flavonoid content of FWE was significantly higher than that of the WE group (P<0.01), reaching 8.42%±0.25%. Both FWE and WE had in vitro antioxidant activity, and FWE was more effective than WE. The C. elegans experiments have shown that the two water extracts have no significant effect on C. elegans fertility (P>0.05), and that the maximum lifespan and resistance to heat stress and oxidative stress of the medium- and high-dose FWE group were significantly higher than those of the WE group (P<0.05), in particular, the maximum life span of the high-dose FWE group in the oxidative stress experiment reached (4.30±0.00) h, which was 16.22% longer compared to the WE group. In addition, the FWE group was superior to the WE group in enhancing Caenorhabditis elegans motility, antioxidant enzyme activity in vivo and reducing reactive oxygen species (ROS) and Malondialdehyde (MDA) levels. In conclusion, the fermentation of Bacillus sp. improved the antioxidant activity of the water extract of PCR and its ability to prolong the lifespan of C. elegans.