Antiviral activity of isatis tinctoria, glycyrrhiza and houttuynia cordata extraction against porcine reproductive and respiratory syndrome virus was estimated in the monolayer Marc-145 cell. The result showed that as for precaution , inhibition and the direct killing function, the best concentration of isatis tinctori were 3.125, 3.125 and 1.563 mg/mL; the optimum concentration of glycyrrhiza were 0.391, 0.782 and 0.782 mg/mL, respectively. The optimum concentration of houttuynia cordata for precaution and inhibition was 0.782 mg/mL, while with on the direct killing function. It indicated that all of these three Chinese traditional medicines had different degrees of antiviral activity against PRRSV and the isatis tinctoria behaved the best in these chinese traditional medicines.in vitro.