In order to clarify the transient variation characteristics of the internal flow in the vertical axial-flow pump device, the computational fluid dynamics (CFD) is adopted to obtain the transient hydraulic characteristics of the blade area of the pump device under various operating conditions, and then the reliability of the numerical simulation is verified by a physical model test. The simulation results show that the area with large impeller inlet bias angle is mainly concentrated on the leading edge of the impeller. With the dynamic change of impeller rotation, the impeller outlet bias angle gradually increases from the hub to the rim, and the area with large guide vane outlet bias angle corresponds to the circumferential position of guide vane trailing edge. Under 1.2Qbep condition, the uniformity of axial velocity distribution at impeller inlet is 89.5%, and the weighted average angle of velocity is 82.9°. When the flow rate increases from 0.8Qbep to 1.2Qbep, the average velocity circulation at the guide vane outlet is reduced by 12.1%. In the range of 0.8Qbep-1.2Qbep, the minimum value of the average velocity circulation is 4.7 m2/s, at which time the flow rate is 1.2Qbep. The vortex structure characteristics at the outlet of the guide vane body is significant. The number of vortex structures from the inlet to the outlet of the guide vane body increases, but the vortex volume decreases, and the resultant velocity of the vortex structure decreases significantly.