Abstract: Large-scale physical model experiments (1:38) combined with three-dimensional (3D) Reynolds stress question model(RSM) numerical modeling are used in this study to explore the hydraulic characteristics in a circular-section bend of high-head spillway tunnel. The 3D RSM is used to simulate the incoming velocities under different pre-designed bend radius and angle conditions, as well as the pressure and velocity distributions in the bend. The simulations compare well with the experimental data. Both occurrence and development of the secondary circulation in the bend are successfully simulated by RSM. An expression for optimizing the minimum bend pressure is derived. The expression establishes the relation between the bend pressure and velocity, and the pre-designed bend radius and angle conditions. The result of using the expression shows that the bend radius is the premier parameter to be considered when designing a circular-section bend of high-head spillway tunnel.