Cavitation in central jet pumps has gained much attention for its effect on transportation efficiency and the safe and stable operation of systems. In order to investigate the mechanism of the cavitation instability in central jet pumps with large area ratios at normal cavitating conditions, numerical simulation has been carried out with Stress Blended Eddy Simulation turbulence model and Zwart-Gerber-Belamri cavitation model. The unsteady simulation predicts the periodic fluctuation of total cavity volume and the time evolution of cavities. Numerical results show that the most important factor for the cavitation instability in central jet pumps is the cavity accumulation induced by the pairing process of cavities. The cavitation instability at conditions with different cavitation intensities is a little different. Moreover, the cavitation period differs a lot at different flow ratios because of different cavitation dynamic behaviors. (C) 2019 Elsevier Ltd. All rights reserved.