The design of inlet sump is an important part in whole project-design. With the aim to investigate the change of inlet chamber affected from impeller under different flows, an inlet sump model with and without impeller was analyzed with numerical simulation method. The results revealed that the fluid field of inlet sump without impeller almost had no change in different flows. The situation in the one with impeller was complicated. The inlet recirculation effect of impeller in part-load had a main influence on the outlet of inlet sump which grew with decreased flow. The distribution uniformity and velocity-weighted suction angle changed down with decreased flow rate. Tangent velocity appeared obviously in the outer of sump outlet at the flow rate of 0.4Q0. It had similar distribution with tangent velocity curve in the vortex and upstream developed in sump. All of these can provide an important reference for design and optimization of large vertical pump unit.