Ultrasonic-assisted method was used to extract flavonoids from Sweet Potato leaves.By the wavelength scan,we found that the maximum absorption peak wavelength of AlCl3-KAC colorimetric method is 360 nm and the maximum absorption peak wavelength of NaNO2-Al(NO3)3-NaOH colorimetric method is 500 nm.The two different blank controls by spectrophotomretry on final the results were compared.The best methods was selected based on standard error,standard deviation,variance,kurtosis and partial degree.The results show that NaNO2-Al(NO3)3-NaOH colorimetric method at the wavelength of 500 nm with distilled water as blank is the best way.This method is reproducible and convenient,and can be used to detect flavonoids contents in other parts of Sweet Potato.