This work is focused on the theoretical investigation of internal leakage of a newly developed pilot- scale fluid switcher-energy recovery device (FS-ERD) for reverse osmosis (RO) system. For the purpose of increasing FS-ERD efficiency and reducing the operating cost of RO, it is required to control the internal leakage in a low level. In this work, the internal leakage rates at different leakage gaps and retentate brine pressures are investigated by computational fluid dynamics (CFD) method and validating experiments. It is found that the internal leakage has a linear relationship with the retentate brine pressure and a polynomial relationship with the scale of leakage gap. The results of the present work imply that low internal leakage and high retentate brine pressure bring benefits to achieve high FS-ERD efficiency.