To study the hydrodynamic performance characteristics of tandem turbines, put forward the reasonable layout scheme and increase power generation efficiency of tidal power station, the transient numerical simulations are performed to investigate a three-blade vertical axis tidal turbine, and the upstream turbine's hydrodynamic performance and the wake effect on the downstream turbine are analyzed. Considering that the turbine's blade has the same profile on the span, the turbine model is simplified to 2D model and the open source fluid mechanics software OpenFOAM is used to simulate the flow field. Firstly,changing law of turbine's power coefficient with tip speed ratio is obtained through the simulation of a single turbine. By comparing with the experimental data in reference,it is verified that the OpenFOAM numerical model can predict the hydrodynamic performance of the 2D vertical axis turbine accurately under the reasonable setting by using the k-ω SST turbulence model and the PIMPLE algorithm. Then two tandem turbines cases are simulated,the distance between two turbines is from 5D to 50D,where D stands for turbine's diameter. The wake characteristics of the sections behind turbines and the influence characteristics of turbines' distance on the downstream turbine's hydrodynamic performance are obtained. Finally the results show that when the distance between two tandem turbines is more than 40D, the effect of upstream turbine on the downstream turbine could be ignored.