Seawater reverse osmosis (SWRO) develops rapidly and benefits from excellent performance of isobaric energy recovery devices (ERDs). Leakage characteristics directly affect the ERD performance. In this paper, leakage characteristics of a fully-rotary valve energy recovery device (FRV-ERD) which is a new type of isobaric ERD are studied numerically. Leakage variation of the fully-rotary valve (FRV) that is the key part of the device caused by pressure difference and geometric parameters is investigated. Then performance of the FRV-ERD is discussed. Results indicate that leakage rates of forward and reverse leakage within the same FRV are almost equal to each other and present a good linear relationship with pressure difference. Leakage rate is exactly proportional to the 3.0 power of the clearance height. Effects caused by changing the length and diameter are equivalent. The leakage rate decreases sharply first and then slowly after the length or diameter is larger than a specific value of which the axial length and the half circumference are equal to each other. The performance of the larger size FRV-ERD is much better than the smaller one, and the required clearance is also significantly increased, alleviating machining difficulty to some extent.