Magnetic fluid has been widely used in sealing gas, but sealing liquids with magnetic fluid is still in the research stage. In this paper, the effect of velocity difference on failure pressure was theoretically deduced. The magnetic fluid seal structure was designed and an experiment its rig was set up, and then the seal performances of sealing gas and liquids with magnetic fluid were studied. The results showed that the failure pressures of sealing liquids and gas statically under the same sealing gap were almost the same, and the failure pressure was generally not affected by the speed of the rotating shaft when sealing gas with Magnetic fluid. The failure pressure decreases significantly when the rotating shaft overspeeded the critical value, under the condition of Magnetic fluid as sealing liquids.