Abstract: The aim is to study the enzyme- assisted extraction technology of triterpenes from Astragali complanati Semen and the antioxidant activityin vitro . Based on a single factor experiment, the four factors of Box-Benhnken experimental design were determined, including liquid to solid ratio, enzyme additive amount, enzymolysis temperature and enzymolysis time. Using the extraction rate of triterpenes as the response value, the response surface method was used to optimize the extraction process of triterpenes in Astragali complanati Semen and a mathematical model was built and analyzed. Antioxidant activity of triterpenes from Astragali complanati Semen was evaluated by free radical scavenging ability system. The optimization analysis showed that the optimal process parameters were as follows: liquid to solid ratio was 40 mL/g, enzyme additive amount was 500 μg/mL, enzymolysis temperature was 40℃, enzymolysis time was 50 min. The theoretical value of triterpenes extraction rate was 8.08%, the actual value was 7.98%, the relative standard deviation was 0.12%. Astragali complanati Semen had a stronger capacity to scavenge DPPH free radical, O2-· free radical, hydroxyl free radical and Fe3+ free radical. Therefore, this method is efficient and simple, and can be used to extract the triterpenes from Astragali complanati Semen; triterpenes from Astragali complanati Semen have significant antioxidant capacityin vitro