Abstract: In this study, physical model tests and numerical simulations of 3-D turbulent flows have been used to optimize the shape of a stilling basin downstream of the tunnel spillway at the Guanyinyan hydropower station through a series of shape modification, and a satisfactory new shape with a backward step and diffusing walls was obtained. Detailed analysis on major hydraulic characteristics, fluctuating pressure in the slab joints, and slab uplift force, shows that protective works should focus on an area at the basin bottom in a distance of 60m behind the step. A significant correlation exists between surface fluctuating pressure and joint fluctuating pressure, and the max uplift force on a unit area of the slabs is 0.4-1.6 times the rms value of surface fluctuating pressure. The test results provide technical support to design and operation of stilling basins.