Abstract: Flow in a Y-type pipe with large diameter difference is complicated and often generates back flows and negative pressure around its branching section, thus becoming a threat to its safety and normal operation. This paper presents a three-dimensional CFD model and simulations of the velocity and pressure distributions in such pipes to improve the fluid patterns in the branching section. Corresponding model pipes are experimentally tested to verify the simulations, and the branching connection is optimized through comparison of its taper and prismatic transition designs. Results show that a taper transition at the branch inlet can efficiently reduce negative pressure zone and reduce back flows around the branching. It plays an important role in prevention of cavitation damage to the pipe and in improvement of the flows.