Abstract: The diversion tunnel of high arch dam located in canyon areasruns through the initial to middlestages of dam construction. Investigating the creation and solution of a diversion risk model of the initial and middle stages of high arch dam construction is therefore important to the design optimization of diversion tunnels. The present work focuses on the engineering design stage and makes use of the dynamic simulation technology of high arch dam construction to collect morphological data covering water retaining and dam protection from the initial to middle stages of construction. We here established a risk model of flow diversion in the initial through middle stages for high arch dam construction considering the factors of hydrology and hydraulic randomness,and a method of solving the risk model using Monte Carlo method,which couples with the water retaining and dam protection morphological data and the major random factors,is proposed in this work. Based on the principle of risk analysis,a risk recognition method for the design of diversion tunnel is proposed,and the mathematical model and detailed steps for the design optimization of diversion tunnel dimensions are given. The results of the case study of a high arch dam in the upper reaches of Jinsha River show that the proposed risk model and the solution method are applicable and effective;the model can be used to determine the risk rate of the initial to middle stages of the entire construction project,and it can objectively reflect the two possible water retaining conditions of the high arch dam during the middle stage of the dam protection period and overcome the limitations of the initial diversion risk model;during the middle stage of construction,the diversion risk rate decreases with the increase in diversion tunnel dimensions;and the feasible scheme set of diversion tunnel dimension designs has a boundary,i. e. the optimal scheme. The outcomes of this study can provide theoretical support for risk decision-making and design optimization for the construction of diversion structures for high arch dams.