Abstract: 【Objective】This study aimed to explore the antiviral effects and mechanisms of Morus alba L. extracts on grouper iridovirus(SGIV), and provide data support and theoretical basis for developing anti-SGIV drugs with Morus alba extracts and offer new ideas for developing efficient and safe antiviral fishing drugs.【Method】The safe working concentrations of M. alba water extracts(MAE) and its active component isoquercetin(IQ) were determined on grouper spleen tissue cell line(GS) by light microscope observation and CCK-8 cell viability assay. Then, real-time fluorescence quantitative PCR(RT-qPCR) and aptamer Q2-based fluorescent molecular probe assay(Q2-AFMP) were used to analyze the antiviral effects of MAE and IQ against SGIV in vitro. RT-qPCR was further applied to detect the expression level of MCP gene and VP19 gene in SGIV infected cells, and explore the antiviral mechanism of IQ, including the effects of IQ onSGIV particles, SGIV bound to host cell surface, SGIV invasion and replication in host cells.【Result】The safe working concentrations of M. alba extracts on GS cells were that, MAE ≤ 10.0 mg/mL, IQ ≤ 1000.0 μg/mL. The research results of MAE and IQ against SGIV showed that, compared to GS cells only incubated with SGIV, the cytopathic effects(CPEs) of GS cells incubated with both SGIV and M. alba extracts(10.0 mg/mL MAE and 1000.0 μg/mL IQ) in the experimental groups were greatly reduced. MCP gene and VP19 gene in experimental group cells decreased extremely significantly(P<0.01, the same below), and the fluorescence intensity of GS cells decreased significantly. The above results indicated that MAE and IQ could effectively combat SGIV infection. GS cells incubated with SGIV treated with IQ showed extremely significant decrease in the relative expression of both the intracellular MCP gene and the VP19 gene, it showed that, IQ could destroy the SGIV particles structure and interfere with the adsorption, invasion and replication of SGIV to host cells.【Conclusion】MAE and IQ have great antiviral effects on SGIV, and IQ plays an antiviral role in the stages of virus adsorption, invasion and replication.M. alba extracts have potentials in the treatments of SGIV infection, and can be used in the development of fishery drugs for inhibiting SGIV infection.