Pressure fluctuation in vertical axial-flow pumping system caused by outlet flow pattern of inlet conduit is the primary sources of operation instability. By means of pressure transduces and red threads installed in outlet section of inlet conduit, fluctuating pressure signals were obtained and recorded under various operating conditions. The signals in time and frequency domains of the signals were analyzed by using short-time Fourier transform method. The results showed that when the flow rate was less than 0.7Qbep, the deviation direction of red threads of internal wall were the same as the impeller rotating direction in inlet conduit, while the flow rate was larger than 0.7Qbep, the deviation direction of red threads of internal wall was along the axial direction. With the increase of rotational speed, the pulsating amplitude of each measuring point was increased at Q<0.7Qbep, while with the increase of rotational speed, the pulsating amplitude was gradually decreased. The pulsating amplitude was decreased firstly and then increased with the increase of flow rate at the same rotational speed, and the pulsating amplitude was the minimum under the optimum operating condition. The main frequency of each measuring points was four times of rotational frequency at different rotational speeds. The subdominant frequency of each measuring points was one time of rotational frequency in Qbep. Both flow rate and rotational speed had significant influence on the subdominant frequency of outlet section in inlet conduit under small and large flow rate conditions. With the increase of rotational speed, the increase amplitudes of measuring point pulsating amplitudes were different under different operating conditions. The pressure fluctuation amplitude was larger in the frequency range of 0 ~ 8 times of rotational frequency for inlet conduit outlet section at different rotational speed.