Abstract: In order to study the hydrodynamic characteristics of the runaway transient process of vertical axial flow pumps, taking Paihekou pump station in the Water Diversion Project from Yangtze River to Huaihe River as the research object, this paper simulated the runaway transient process of the vertical axial flow pump by using numerical simulation method, which combines the Fluent 16.0 software and UDF secondary development program. The variation of external characteristic parameters such as speed, flow rate, torque, axial force, and the evolution characteristics of streamline in elbow inlet conduit during the runaway transient process are obtained by numerical calculation. The results show that the runaway speed calculated by numerical calculation is 218.99 r/min under average head, and the relative error is 1.69% compared with runaway speed of 215.33r/min that is converted by model test. The maximum axial force of the impeller is 225.93 kN during the runaway transition, not greater than the impeller weight of 342.12 kN and the direction of the axial force is opposite to the direction of gravity. There is no “lifted” accident for pump units and a spiral vortex occurs in elbow inlet conduit during the runaway transient process. The research results provide references for the design of vertical axial flow pumps.