A series of flume experiments were carried out to investigate the influence of tidal turbine rotor diameter to depth ratio on the hydrodynamic process of wake flow using two different diameter rotor discs of the same porosity. Time-varying velocities were measured by an Acoustic Doppler Velocimeter at 8 cross-sections over the distance of 10 diameters downstream, and the three-dimensional structures of wake flow and turbulence fields were obtained. Immediately downstream, the peak of velocity deficit occurred at the wake core and the value was greater for the large diameter-depth ratio. Strong wake turbulence was mainly located in the shear stress layer around wake core. However, the attenuation processes of wake hydrodynamics were different under two diameter-depth ratios. The momentum transfer was caused by Reynolds shear stress in both transverse and vertical directions. The vertical momentum transfer process was much more significant above the wake core than the lateral transfer process, but it decayed rapidly in near wake as the diameter-depth ratio enlarged. The experimental results provide detailed data to better understand the wake propagation processes behind rotor discs.