Abstract: Aiming at the spillway tunnel radial gate structure of Baihetan Hydropower Station with large width and height ratio, this paper uses the first horizontal three-arm hidden radial gate type in China. The multi-factor analysis of key design parameters, such as the placement of the three arms, the unit stiffness ratio of the main beam to the arm, and the finite element numerical calculation verification under multiple load conditions are adopted. The optimal arm length and unit stiffness ratio of the radial gate of Baihetan spillway tunnel are determined.Furthermore, the force distribution of the horizontal three-arm radial gate is optimized, which effectively guarantees the bearing safety of the civil structure under the huge discharge. The results show that the optimal arm length and unit stiffness ratio in Baihetan are 2.5 m and 10.7based on the criteria for determining the length and unit stiffness ratio of the three lateral arms on the basis of the sensitivity analysis of multifactor parameters. The main calculated stress of the optimized radial gate structure is less than 145 MPa, and the maximum calculated combined displacement is less than 15 mm, both of which meet the requirements of strength and stiffness. The key technology of horizontal threearm radial gate is researched and established, which can provide technical support for the safe operation of spillway tunnel with super widthto-height ratio orifice and super large discharge volume.