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This research was taken on probing the internal flow pattern of relative velocity flow field in the back-swept double blades centrifugal pump. Firstly, different turbulence models, including k-ε , SST and DES were used to do unsteady numerical simulation on the particle image velocimetry (PIV) test pump whose specific speed was 70. Secondly, PIV test was done on the pump and the relative velocity flow field distribution in the impeller was got. The results showed that numerical simulation results of k-ε turbulence model were more consistent with PIV test results. By analyzing the distribution of relative velocity of the impeller whose specific speed was 70 under different flow rate conditions ( Q/Q d was 0.6, 1.0, 1.6), the variation of axial vortexes and low-speed zones in the impeller was found out. When Q/Q d was 0.6 and 0.8, there were low-speed zones and axial vortexes were opposite to rotation directions of impeller in the middle zone of channels nearby the pressure surface of the blades, and with the increase of flow rate, low-speed zones and axial vortexes decreased. Then k-ε turbulence model was used to do numerical simulation on the pump whose specific speed was 157, the results showed that the variation law of the relative velocity flow field distribution in the impeller was similar with that of PIV test pump. To explain the phenomenon, the theory of limited number blades in the centrifugal pump internal flow was introduced, which revealed the fundamental reason for the existence and development of the low-speed zones and vortexes. The research results have an important reference value for the further research of the internal flow pattern of protrusive type twisted blades centrifugal pump.