This invention provides animal powered mechanical device for water desalination. It has unique features of using animal power as prime mover to effect brackish water desalination. Animal energy in form of high-torque low-speed can be converted into low-torque high-speed through speed increaser (4) to energize the high-pressure pump (6), the outlet manifold (8) of the pump (6) being connected to a reverse osmosis membrane module (9). With this device animal power can be used for obtaining safe drinking water from backish ground water/seawater resource in villages.