Waterjet pumps generally use axial flow pumps or guide vane mixed flow pumps. A hump or even double-hump are often observed in the Capacity-Head (Q-H) curve for the high specific speed mixed flow pumps and axial flow pumps. When the pump is running in the hump region, the pump and the entire system may have problems of flow turbulence and increased vibration and noise. The waterjet pump may enter into the hump region during the rotation change of the waterjet pump corresponding to the ship speed change, which imposes hindrances to the efficient and low-noise operation of the ship, and affects the comfort of the crew and the stealthiness of the ship, even damages the propulsion system. The hydraulic performance curve of a test pump for a mixed-flow waterjet pump with guide vanes is obtained based on the standard test platform of axial (mixed) flow pumps and high-precision test method. It can be observed that there is a significant hump in the energy characteristic curve, accompanied with intensified vibration. A three-dimensional model of the fluid domain has been established according to the geometric model of the test pump for the waterjet pump, being divided by refined structured grids. The results show that under the trough condition, there are significant low-speed vortices in the impeller domain at a higher height in spanwise. The structure and scale of the low-speed vortices between different flow channels are different. In region A, though the flow rate is relatively slow, the flow still moves forward in a spiral. And in region B, the flow rate is even lower but with greater degree of rotation. Although the flow velocity in the A region is lower but still spirals forward. The flow rate in region C is slightly larger than those in region A and region B. However, under the crest condition, the flow in the impeller domain is more uniform and smooth, and the stable flow structure in the flow channel is significantly reduced.