Abstract: Abstract: In order to study the characteristics of the flow field and gas-liquid two-phase flow hydrotransport in vortex pumps, the performance test and the measurement of flow field in volute at optimal operating condition with five-hole probes were conducted on a self-built vortex pump (32WB8—12). Based on these experiments, the qv-H, qv-P, qv-η, qv-NPSHc performance curves and the absolute velocity v, circumferential velocity vu, radial velocity vr, axial velocity vz, and the flow static pressure ps, on the 5 measuring points were obtained. The obtained parabolic qv-NPSH performance curve shows the opposite tendency compared to the centrifugal pump. The mechanical properties of the four kinds of forces in gas-liquid two-phase flow were analyzed with the Navier-Stokes Equation, and the results indicated the constraint relationship between performance and mechanical factors. Furthermore, the principle of gas-liquid flow hydrotransport and the difference between cavitating current and gas-liquid flow were also presented. The interior flow field in the pump under the optimal condition has been numerically simulated by FLUENT software based on the renormalization group k- ε turbulence model (RNG k- ε model) through which the pressure and velocity as well as gasvolume distribution of three cross sections in the vortex pump were obtained. The experimental results also showed that axial vortex was the dominant flow in the pump inlet, and the airflow gathered in the volute inlet with low pressure. The difference of molecule size, shape, and elastic modulus between airflow and liquid leads to the variations of the inertia, friction, and buoyancy forces in flow field, which explains the relationship between performance and parameters of the internal flow and gas-liquid suction as well as the head formation principle of vortex pump. The numerical results are in good agreement with flow field measurement data. The results show that the internal flow in the vortex pump mainly consists of through flow. The flow in the impeller region is of forced vortex characters, and the flow in volute is similar to the combined vortex with backflow, which is a non-axisymmetric unsteady flow with quite high turbulence intensity. All these should be main reasons for the relatively low efficiency of the vortex pump. The distortional principle of velocity field and the channel streamline with two-phase flow inside the vortex pump have been investigated in this article. This research provides a model of gas-liquid two-phase flow and cavitating current in vortex pump.