The water extract was obtained by aqueous leaching in low-temperature from Cordyceps sinensis mycelium.The toxicity and antiviral activity of water extract were investigated by using MTT method and observing cytopathic(CPE),respectively.The results showed that the concentration of water extract≤ 0.5mg/ml was no toxicity on Vero cells.Either pre-mixed the water extract with newcastle disease virus or with Vero cells for 1 hour before cell cultivation assay was able to inhibit cytopathic effect of newcastle disease virus significantly.The research indicates that the water extract has direct and preventive effect against newcastle disease virus infection.