On the basis of model test, a 3 D numerical simulation on the flow field of low waterhead and big discharge riverbed hydroelectric station was made by adopting k-ε turbulence model and VOF method. The calculated flow pattern, water surface line,flow velocities ,and the pressure distributions were compared with test results. The results show that the calculated results are in good agreement with the experimental data. It is proved that the turbulent mathematical model , the basic control equation and discrete method are reliable. The results can be used for the engineering design.