The experiments on changing the axial location of impeller and measurement of flow field in volute with five-hole probe were conducted on a self-built vortex pump (32WB8-12). Based on the experiments, the qv-H, qv-P, qv-η, qv-NPSHc curves and the absolute velocity v, the circumferential velocity vu, the radial velocity vr, the axial velocity vz and the flow static pressure ps were obtained. The experimental results proved that the qv-NPSHc curve showed opposite tendency in the operating conditions of small charge compared with centrifugal pump and anti-cavitation were improved with the increase of the scale S that impeller was inserted into volute. The investigation also showed that the axial vortex was dominant flow in volute but with back-flow existing at the same time. The relationship between performance and parameters of the internal flow and the suction, head formation principles of vortex pump was explained. 