In order to explore mechanism and propagation characteristics of rotating stall, the unsteady flow characteristics of a mixed-flow pump are calculated based on SST turbulence model. Based on pressure fluctuation characteristics, number of stall cells and circumferential propagation velocity are calculated accurately under different stall conditions. With the circumferential evolution and transfer of stall cell, transient head shows a large periodic fluctuation. Pre-stall stage with weaken dissipation always occurs in the clockwise adjacent path of stall path. When a path is seriously disturbed by stall cell, the blocking effect will change the flow structure of rim flow and intensify the attack angle of downstream path, resulting in the formation of tip initial stall. Then the initial stall cell gradually propagates downstream, and experiences stages of development and attenuation. The fundamental reason for stall propagation is that the existence of stall vortices changes the trajectory of some fluids. The changing trajectory of rim flow is concentrated performance. Hence, distorted rim flow is the main inducement of pre-stall and tip initial stall. Through stall propagation mechanism, stages of flow paths at different time periods can be determined. The results could serve as reference to discover the stall mechanism under critical stall point. (C) 2020 Elsevier Ltd. All rights reserved.