The unsteady numerical simulations were carried out to investigate the transient flow features of a centrifugal pump system at startup and valve-opening processes. The scale-adaptive simulation (SAS) based on SST turbulence model was used in the simulation. The computational domain was the entire closed double-suction centrifugal pump test rig. The transient head of the pump, the pressure pulsation on the casing and the radial force on the impeller were focused on. The results showed that the predicted transient head of the pump in startup processes agreed well with the experimental data. Compared with the steady-state calculations, the transient valve-opening simulation produced a pump head which was more close to the experimental data. The transient flow rate increased significantly with the increase of valve-opening time, while valve-opening speed almost did not affect the transient head. During the valve-opening process, the pressure pulsation on the volute changed cyclically, the dominant frequency equaled to the blade passing frequency. In addition, the most dramatic change in pressure pulsation amplitude was found in the region near the tongue. The radial force on the impeller decreased with the increase of the valve opening size, and it changed with the blade passing frequency.