The optimized extraction conditions of total flavonoids from mimenghua were determined by the Box-Behnken design and response surface methodology. The optimal conditions to achieve the maximum yield of flavonoids were determined as follows: ethanol concentration 68%(v/v), extraction temperature $72.4^{circ}C$, liquid/material ratio 48:1mL/g, and extraction time 2 h. Under the optimized conditions, the extraction ratio of flavonoids was 15.44%. The crude extract was then purified by AB-8 macroporous adsorption resin, the flavonoids content in the purified extract increased to 90.43%. The antioxidant activities of the purified flavonoids were evaluated in vitro by scavenging capabilities of DPPH radical, hydroxyl radical and superoxide radical, reducing power and total antioxidant capacity. The results showed that the flavonoids have significant antioxidant activities, which can be used as a source of potential antioxidant.