Energy recovery device is now an essential apparatus for SWRO plants, and it can be classified into two categories: the turbine type and the positive displacement PD type. The PD type contributes a higher energy recovery efficiency (ERE) (> 90%) than the turbine type (50%-75%) and becomes a more promising and competitive technology of the field. The experiment simulates the PD energy recovery process under a lower test pressure/HP concentrate pressure (similar to 0.6 MPa). Mainly, the dynamic characteristics and effect of HP concentrate pressure on the ERE of the unit are inspected and analyzed.