To investigate the effects of opening time of vacuum breaking valve on transient hydraulic characteristics of a mixed-flow pump unit during the shutdown process, a full-flow geometric model of the pump unit was established. The actual vacuum breaking valve device was simplified into a ball valve model with equivalent flow area, and the coupling of ball valve opening-closing motion and impeller rotation was realized by the dynamic mesh technology. Three schemes with different valve opening times were designed, namely the advanced opening scheme, the synchronous opening scheme, and the delayed opening scheme. Three-dimensional numerical simulations of the shutdown transient process were carried out. The influence mechanism of valve opening time on hydraulic stability during the shutdown process was explored from three aspects: external performance, pressure fluctuation characteristics, and internal flow characteristics. Combined with the entropy production theory, the energy dissipation law during the shutdown process was revealed. The results show that the external performance parameters, pressure fluctuation and internal flow field evolution present similar variation tendencies under different valve opening schemes, while differences exist in temporal evolution process and variation amplitude. The opening time of the valve exerts remarkable impacts on the transient performance of the unit. Compared with the synchronous opening scheme, the advanced opening scheme reduces the peak values of torque and axial force, yet it may induce more severe initial hydraulic impact. In contrast, the delayed opening scheme delays the operating condition transition, which makes the reverse extreme values of torque and axial force increase to 1.26 times and 1.21 times those of the synchronous opening scheme, respectively, and amplifies the pressure fluctuation amplitude at the siphon crown. Moreover, the delayed opening scheme significantly increases the hydraulic energy loss in the impeller region. In summary, the synchronous opening scheme can effectively suppress hydraulic oscillation, reduce load impact and energy loss while maintaining favorable shutdown efficiency, which is the optimal strategy for stable shutdown of the pump unit.