Tidal stream energy has been considered as one of the most promising renewable and clean resources to ease energy shortage. This study experimentally investigates wake characteristics of a tidal stream turbine subject to combined wave and current, using an advanced acoustic Doppler velocimeter to measure its wake properties. The phase-space filter is used to remove spikes in raw data, and the phase-averaged method is adopted for further data analyses. The experimental results indicate that the mean wake characteristics under combination of wave and current share a similar trend with that under pure current. However, it's found that the wave-current interaction fastens the wake recovery and increases the turbulence intensity level. Besides, the supporting structure is found to play an important role in the near wake. A region with low velocity and high turbulence intensity is observed in the lee side of the supporting structure. Furthermore, the wake zone is found to shift upward to the free surface due to wake merging of the turbine rotor and the supporting structure.