The stress distribution around the orifice of high arch dam is complex, while proper reinforcements is necessary to improve the stress state of the orifice structure and ensure safe operation of arch dam. Taking the No. 3 bottom orifice of the Baihetan Arch Dam as an example, a 3-D finite element calculation was firstly made on the orifice structure under construction condition for analyzing the the stress distribution and mechanism around the orifice. Then the stress graph method was used to obtain a reinforcement design of the orifice. Finally, the non-linear calculation of reinforced concrete was carried out for the orifice with reinforcement. The results showed that the Poisson's ratio effect of concrete was the main cause of the tensile stress along the river at the top and bottom of the orifice under the action of gravity of dam body. The overall stress of the reinforcement was not high, and the reinforcement design scheme had a great safety margin, which means the stress graph method is conservative. However, the stress graph method should still be adopted for the reinforcement design in terms of safety.