The transient cavitation flows in centrifugal pump for small flow rate are numerically investigated using RNG k-ε turbulence model and transport equation cavitation model, which are modified by taking the influence of the compressibility of fluid and the effect of pressure fluctuation on saturation pressure into condition. The calculated variation of pump head with pump inlet pressure agrees well with the experimental data, which demonstrates that the numerical model and method can accurately predict the cavitation flows in centrifugal pump. The characteristics of pressure fluctuation with and without cavitation are compared and analyzed. The dominant frequency of pressure fluctuation in impeller is the rotating frequency. From the impeller inlet to outlet, the maximum amplitudes of pressure fluctuation in flow passage and blade suction side gradually increase. But the maximum amplitude reaches the largest value at the 4/5 length of the blade from the blade inlet side for the blade pressure, which is induced by the flow separation in this area. The maximum amplitude of the cavitation flow is larger than the non-cavitation flow, about 2 times at the flow passage inlet. With the influence of the asymmetric structure of volute tongue, the distribution of cavitation in impeller is not uniform.