Abstract: The back-swept double blades sewage pump studied in the paper belongs to the new high efficiency non clogging pump. It has the self-cleaning ability and can effectively solve the problem of fiber winding and congestion. This research was taken on probing the internal flow pattern of relative velocity flow field in the back-swept double blades sewage pump, of which the specific speed was 70. The main design parameters of the pump showed as follows: design flow rate Qdes = 17 m3/h, head H = 9 m, rotation speed n = 1 450 r/min. Performance test was done to get the performance parameters curves and results showed the highest efficiency is 61.9% when Q = 21.1 m3/h, corresponding to H = 8.2 m. Then PIV test was done and the field distribution for relative velocity flow in the impeller was got. The test PIV system was the US TSI company's commercial PIV system, including: YAG200-NWL type pulse laser; 610035-type synchronizer; 630059POWERVIEW 4MP type cross-frame CCD camera; embedded Tecplot software for image acquisition and data analysis systems Insight 3G; 610015-SOL type light arms and light sheet lens group. External trigger synchronization system comprising: a shaft encoders, flexible couplings, synchronous trigger controller and optical fiber transmission converter. By analyzing the distribution of relative velocity of the impeller in different flow rate conditions (Q/Qdes = 0.4, 0.6, 0.8, 1, 1.2, 1.4), we found the variation of axial vortexes and low-speed zones in the impeller. When the flow rate changes from small to large, low-speed zones decrease, and the axial vortex zones were also reduced. When the flow rate changed from Q/Qdes = 0.4 to Q/Qdes = 0.6 which means the test the pump was at low flow rate conditions, there were large low-speed regions, axial vortex zones, and the flow field was very unstable, which meaned large hydraulic losses. These axial vortices scattered pressure sides of blades near the inlet, and spreaded to the back of the blade. When the flow rate changes from Q/Qdes = 0.6 to the Q/Qdes = 1.4, the low speed area gradually reduced. Overall, when Q/Qdes = 0.6 and 0.8, there were low-speed zones and axial vortexes 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 decreases. To explain the phenomenon, the theory of limited number blades in centrifugal pump inner flow was introduced, which considered the relative speed of liquid in rotational impeller as the composition of axial vortex movement and fixed tubular flow. It revealed the fundamental reason for the existence and development of the low-speed zones and vortexes. Meanwhile, we analyzed the relative velocity distribution of different relative phase position between the impeller and volute when Q/Qdes = 0.6, and studied the effect of interference between the impeller and volute on the axial vortexes. It could be known that when the axial vortexes flow through the volute, the vortexes shift to the downstream of the channels. The research results have an important reference value for the further research of the internal flow pattern of protrusive type twisted blades sewage pump.