Abstract: For a hydropower plant, strong pressure fluctuations, vibrations and water hammer could occur during a transient process, resulting in negative effects on the safety and stability of its operation. To capture the dynamic properties of a turbine at the transient stage, we have adopted the CFD Fluent 14.0 and a dynamic mesh method on the basis of finite volume method, and simulated the three-dimensional transient flows in the guide vane and runner passages of a Francis turbine. This method focuses on simulation of the internal flow changes in the runner caused by large-fluctuation transients and analysis of the corresponding changes in velocity and pressure fields. Thus it is distinguished from the conventional steady-state analysis methods. The calculation results indicate that the dynamic mesh method is effective and successful and it provides a new tool for the study of three-dimensional flow transients.