An experimental investigation on fluctuating turbulent flow with different amplitudes, frequencies and mean values of flow rate in a narrow rectangular channel was carried out to determine the phase difference so as to find out real corresponding relationship between pressure drop and flow rate. It is found that the measurement delay time difference between the flow meter and the differential pressure transmitter is not a constant but vary with the different flow rate fluctuation conditions. The phase difference was calculated by a function which is given in this paper and tested by the result of two kinds of nonlinear fit methods, whose results agree well.