The filter based model (FBM), which was introduced into CFX by user defined subroutine, and a homogeneous cavitation model was used to calculate cavitating flow of the axial-flow pump at different conditions. The effect of a maximum density ratio between liquid and vapor in the cavitation model on cavitation characteristic simulation was investigated. The results compared with experimental data show that the FBM based on RNG k-εmodel can predict the external characteristic of axial flow pump well. The critical NPSHs predicted by the true density ratio 43197 at three different conditions (Q=320, 392, 480m3/h) are closer to experimental values than the default 1000 and the relative errors are 1.93%, 4.73% and 8.09% respectively. With the increasing of maximum density ratio, the calculated vapor area, vapor volume fraction and the range of tip leakage vortex increase as well. This may be the cause of different predicted critical NPSHs.