A combination large gap ferrofluid rotation sealing device of magnetic fluid and magnetic lubricating grease was designed for the sealing of the cooling medium of high power electric motor of large vessels equipment,which has five stage and nine tooth,and is suitable for large ships with high rolling and corrosive environment.The pressure formula of the ferrofluid rotation sealing device were derived theoretically.It is concluded that the pressure capacity is related with saturation magnetization of the ferrofluid,the yield stress of the magnetic grease,the magnetic field gradients in the sealing gap and other factors.The magnetic field distribution within sealing gap for the designed ferrofluid rotation sealing device was analyzed by using ANSYS.The pressure experiments were carried out on the sealing bench.The experimental results show that when the maximum gap is 0.7 mm,the singlestage pressure capacity can still reach 18 kPa,and the sealing ability keeps stable with the increase of rotational speed.The theoretical and experimental results show that the designed seal is suitable for large clearance sealing of highpower motor or other highvibration equipment in corrosive environment.