In order to examine the impaction of different initial phase angles on the hydrodynamic performance of vertical axis tidal twin-turbine with different densities, this paper used CFD numerical simulation method to obtain the variation law of efficiency curves, and then explained the change mechanism through the detail analysis of the pressure diagram and vorticity graph of different density vertical axis tidal turbine with different phase angles. The results show that the influences of the phase angle on different density twin-turbine are similar. With the increase of phase angle, the efficiency of unit 1 with middle and high ratios is reduced gradually, unit 2 is the exact opposite of unit 1; When different density tidal turbine works at the respective middle phase angle, the efficiency of unit 1 and unit 2 of almost the same, which benefits the power delivery. Moreover, middle phase angles minimize the pressure of units. So the respective middle phase angle should be chosen as the optimal phase angle for different density tidal twin-turbines.