Abstract: The phase lags in pulsating single-phase laminar flow of rectangular channels with big aspect ratio were studied. The results show that the low frequency flow rate pulsation does not arouse the distortion of velocity profile. There is a phase lag between the pressure drop and flow rate, and the phase lag is related to the narrow edge length of the channel, fluid viscosity and the pulsating period. The bigger the pulsating period or the viscosity is, the smaller the phase lag is. The longer the narrow edge is, the larger the phase lag is. In addition, the pulsation amplitude is not related to the phase lag. The phenomena are discussed based on the mathematical model.