The influence of transition section shape from outlet sump of pumping station to free-flow tunnel on current capacity, surface line and local head loss is significant. Aiming at solving the problem that the flow characteristic of a pump station is complex, the transition section was lengthened and warped surface wall was optimized by a test model with a geometric scale of 1/25 in accord with. The results show more smooth streamlines and an increase in flow capacity in the modified transition, along with a reduction from 0.051~0.074 to 0.051~0.074 in local head loss and a reduction in ranges of central longitudinal water surface profiles. The hydraulic characteristics indicators show that the transition section after optimization performs a better characteristics compared with that of the original design scheme. The optimal design of the project can provide a reference for other similar projects.