Abstract: In order to study the vortex characteristics of internal flow inside the two-stage double-suction centrifugal pump, the unsteady numerical simulation method based on the SST k-ω turbulence model was adopted. The rationality of the numerical calculation method was verified by the performance test and GCI criterion. A new generation vortex identification method—Ω method was introduced to analyze the vortex structure of internal flow field. The results show that the traditional vortex identification method cannot effectively eliminate the influence of wall shear layer, while the Ω method can distinguish the shear and pure rotational motion of the fluid, and obtain a clearer vortex structure in the pump, especially in the two-stage impeller, which can accurately capture the morphology and distribution of the wake vortex. Under different working conditions, after simplifying the three-dimensional Ω vortex identification method to the two-dimensional plane, it has better adaptability in the back channel, and it circumvented many regions where the flow line showes strong distortion without forming vortex structures before, and it also proves that axial vortex structures is dominate in the back channel. In addition, the research process proved that the Ω method has a greater advantage in identifying the internal vortex structure of complex fluid machinery such as a two-stage double-suction centrifugal pump, which can provide a relevant theoretical basis for further design optimization of this type of pump.