Abstract: A new method for measuring the frequency characteristics--the quadraturemultiplication method, is set forth. Theoretical analysis shows that the advantage of the quadrature multiplication method over the conventional methods lies in that it can markedly reduce the error in the frequency characteristics of the rejection band, so that it can extend the measurable extent of the rejection-band attenuation. Actual tests verified that almost no error could be found even when the relative attenuation reached 70 dB.