Abstract: To study the flow characteristics of pulsating single-phase laminar flow in circular pipes, a mathematical model was established to study the phase lag which is exist between the pressure drop and flow rate. The results show that the low frequency flow rate pulsation does not arouse the distortion of velocity distribution; however, there is a phase lag between the pressure drop and flow rate, and the phase lag is related to the radius of the pipe, fluid viscosity and the pulsating frequency. The bigger the pulsating frequency or the radius is, the larger the phases lag is. In addition, the phenomena were discussed based on the mathematical model.