Abstract: Abstract: A vortex pump is totally different from the centrifugal pump in physical and flow structure. Since the current vortex pump is developed from the centrifugal pump, many structural parameters cannot be suitable for the vortex pumps. Therefore, it is necessary to determine the structural parameters of the vortex pump in particular. In this study, five groups of volute models with different tongue angles were designed to clarify the effect of volute tongue angle on the performance and unsteady flow characteristics in a vortex pump. The numerical calculation was performed on the vortex pump using the commercial software ANSYS CFX. The full flow field of the vortex pump was simulated using the Navier-Stokes equations and the RNG k-? turbulence model. The energy test and pressure pulsation test were conducted and the test results were applied to verify the numerical simulation method. The energy performance showed that there was an optimal angle for the best pump performance. The flow field showed that the volute tongue angle posed a great influence on the flow state of the volute tongue and the diffuser section. Specifically, the relatively smaller tongue angle significantly reduced the flow area of the volute throat, and then blocked the rotational movement of the fluid in the non-vane cavity for the higher dynamic and static interference between the circulation flow and the volute tongue. By contrast, the larger tongue angle caused the large-scale vortex and backflows in the diffuser section. Furthermore, the tongue angle and the position of the measuring point also determined the distribution characteristics of pressure pulsation at the volute tongue during operation. The pressure pulsation at the volute tongue decreased first and then increased with the increase of the tongue angle. The pulsation amplitude at twice the axial frequency was significantly higher than that at other angles when the volute tongue angle was 30°. After that, the pulsation amplitude at twice the axial frequency decreased significantly at the volute tongue angle of 40° and 45°. Once the volute tongue angle reached 50°, there was a great increase in amplitude of low-frequency pulsation at 0.25-0.5 times the axial frequency. Meanwhile, the pressure pulsation of all frequencies decreased gradually with the increase of the axial distance between the measuring point and the impeller. The vortices field demonstrated that the pressure pulsation with twice the axial frequency at the volute tongue was produced by the development and diffusion of spiral inlet flow. The asymmetry pressure distribution was attributed to the vortex core distribution at the volute tongue. Consequently, the higher tongue angle can be expected to effectively improve the stability of the internal flow at the volute tongue of the vortex pump for a higher head and efficiency than before. The best volute tongue angle of 45° was also achieved for the performance of the improved model. This finding can also provide a promising theoretical reference to optimize the vortex pump.