The invention provides a group delay method to test the group delay Tgd. It includes the steps: (A) inputting a known period-T analog single-frequency signal to one input end of the tested device; (B) extracting the input single frequency and the output delayed one and converting them into a first digital signal and a second one, respectively; (C) comparing phase difference of the first and second signals; (D) converting the comparing result into a current I in direct proportion to the phase difference; (E) making the current I flow through a average circuit with a value-known resistance R and obtaining a voltage differenceªñV; and (F) according to known T, I, R andªñV, finding Tgd.