The rotary type energy recovery device (RERD) has been widely employed in the Reverse Osmosis membrane technology to decrease the system electricity consumption. Understanding of the flow dynamic in the rotor duct is beneficial to design a high-performance of a RERD, but visualization experiments have hardly been carried out due to the high-speed rotation of the rotor duct. Hence, a visualization apparatus of a RERD with a single stationary duct and two rotary endcovers was proposed in this study. A two-dimensional particle image velocimetry (PIV) measurement was applied to visualize the flow structure in the duct. The phase-locking method and the time-averaged processing method were utilized to analyze the vortex dynamics and the flow characteristics. From experimental results, an "entrance effect" can be clearly identified at the duct entrance, which caused the vortex formation. At the duct entrance, the unstable flows structure demonstrated a three-dimensional structure. The liquid piston, a stable flow with uniform velocity, also can be found in the middle section of the duct, effectively preventing the over mixing between the seawater stream and the brine reject stream. These experiment results would be beneficial to researchers in better understanding the flow dynamic in the duct of a RERD.