In the design of structure of spillway tunnel with free water surface, design discharge and check discharge should be fulfilled. In view of this, we established a numerical flow model for Yellow River diversion project of Xiaolangdi, located at the right bank of Jianhe River in Yuanqu of Shanxi province. On the basis of volume of fluid (VOF) model and Mixture model, we obtained relevant parameters including discharge capacity, distribution of water depth along the direction of water flow, cavitation number, sectional velocity , pressure and ski-jump distance on the conditions of design water level and check water level. The results show that, simulated values are in good agreement with test values; and it is feasible to study hydraulic characteristics of spillway tunnel with free water surface by using Fluent software; finally, the research results can provide theoretical reference for the application of the software in practical projects.