Abstract The flow and sound field in a vertical sump centrifugal pump were solved by using an indirectly mixed computational method of computational fluid dynamics(CFD)software ANSYS CFX15.0 and acoustics simulation software LMS Virtual.lab to investigate unsteady characteristics and noise pattern of the pump. In this method, firstly, the unsteady pressure pulsations in time domain at a few monitoring points in the flow field were obtained by solving the flow field, thereby the characteristics of these unsteady pressure pulsationsin frequency domain were determined. Based on the acoustic boundary element method, the internal acoustic field noise of the volute and blade dipoles was solved, the sound pressure level distribution on the surface of boundary elements and the sound pressure frequency curve at typical field points were obtained. The results showed that the interaction between sweeping blades and the volute tongue produces a larger pressure pulsation, and the noise near the tongue is the main sound source for the flow borne noise.The sound pressure level reaches the maximum at blade passing frequency and its harmonic frequencies.With increasing frequency, all the maximum sound pressure levels decrease. The results presented in this paper can provide a certain theoretical basis for subsequent analyses of noise reduction issued by vertical sump centrifugal pumps.