The total flavonoids were extracted from Paulownia fortunei flowers by ultrasonic-assisted cellulase method. Based on the single-factor test, enzymolysis temperature, ultrasonic power, liquid-solid ratio and enzymolysis time were selected for the Box-Benhnken test design. The extraction process was optimized by response surface methodology and the antioxidant capacity was determined. The results showed that the optimal extraction conditions were enzymolysis temperature 50 ℃, ultrasonic power 240 W, liquid to solid ratio 40∶1 (ml∶g) and enzymolysis time 36 min. Under the optimal extraction conditions, the extraction yield of total flavonoids was 7.82%, which was close to the model prediction value of 8.01%. The results of the antioxidant test showed that the half inhibitory concentration (IC50) of P. fortunei flowers total flavonoids on DPPH radicals, OH radicals and ABTS radicals were 214.2 μg/ml, 200.7 μg/ml and 328.5 μg/ml, respectively. With the total flavonoid concentration of 800 μg/ml, the clearance rates of total flavonoids to the three free radical were 83.1%, 75.4% and 64.3%, respectively.