Largemouth bass ranavirus (LMBV) is widely prevalent in China, seriously threatening the safety of largemouth bass (Micropterus salmoides) aquaculture. Chinese herbal medicines have become important for screening LMBV prevention and treatment drugs. Many natural medicine plants have been shown to have direct or indirect antiviral effects. Natural medicine plants can directly impair the virus architecture, hinder the virus invasion, or block the virus entry. In multiple cases, natural medicine plant ingredients have been confirmed to inhibit the virus by stimulating the host’s innate immune responses. Cirsium japonicum has many biological activities such as antivirus and anti-inflammation properties. The anti-LMBV effect of Cirsium japonicum aqueous extract (CJAE) was analyzed in vitro and in vivo to investigate the inhibitory effect on LMBV. CCK-8 tests determined that the safe concentration of CJAE on Siniperca chuatsi brain cells (SCB3) was ≤1000 μg/mL, and CC₅₀ value was 1487.0 μg/mL. The qPCR and indirect immunofluorescence assay showed that CJAE could effectively inhibit the synthesis of viral nucleic acid and MCP protein in SCB3 cells, and the EC50 value was 457.3 μg/mL. Moreover, the inhibition rates of CJAE on LMBV invasion and replication were 98.46% and 99.31%, respectively. CJAE was added to the basal diet at 0.5%, 1.0%, and 1.5% to feed largemouth bass twice daily for 15 days. Then a challenge test (1×10⁶ TCID₅₀ LMBV) was conducted. Feeding CJAE could provide 35%–60% RPS and reduced the viral load of spleen, liver, kidney and intestine of largemouth bass by 87.57%–99.99% through tissue virus copy number determination, which differed significantly from that in the control group (P＜0.01). CJAE increased the phagocytic activity of head kidney leukocytes by 35.88%–60.75% through phagocytosis assay, and activated the expression of phagocytosis-related genes (FcγRIa, CFL and Lyn) through mRNA expression determination; CJAE significantly improved the inflammation of largemouth bass caused by LMBV infection through tissue H&E staining. Further studies showed that CJAE significantly activated the expression of anti-inflammatory factors (IL-10) and inhibited the expression of pro-inflammatory factors (TNF-a and IL-1β). This study explored the anti-LMBV activity and underlying mechanisms of CJAE and supplied new insights for screening anti-LMBV drugs. This study provides a basis for using CJAE to prevent LMBV infection in production and further analysis of anti-LMBV active ingredients in CJAE.