In order to solve the problems of large prediction deviation caused by multiple factors affecting the discharge capacity of the diversion tunnel and the complex action mechanism,we identified and classified the influencing factors of the discharge capacity of diversion tunnel by integrating theoretical analysis,numerical simulation and model test.By analyzing the mechanism and degree of influence of each factor,we proposed the method of reducing the prediction deviation of discharge capacity.Roughness simulation deviation and scale effect of the modeled water-air two-phase flow are key factors that lead to the large deviation of the predicted discharge capacity,resulting in deviations of the open and full flow in cavern and the flow rate interval,which makes the open and full flow transforms to full flow in advance.To reduce the deviation,the range of the open and full flow obtained by test needs to be corrected.Under the condition of pressure flow,the roughness simulation deviation can be calculated by using the roughness deviation value of the original model according to the steps introduced in this paper.