Abstract: The precise computation for the seal capacity of magnetic fluid seals is a rather complicated problem, because this technique concerns with the fundamental theory and analysis method of tribology, electromagnetics and hydrodynamics, and the computations of the seal capacity should be based on the magnetic field computation. Thus, the magnetic field model is analyzed and simplified. On the basis of magnetic fluid seal theory, the magnetic field and isobars in the magnetic fluid are computed with finite element method. The seal pressure differential of the seals is worked out. The relation of the seal pressure differential with the magnetic fluid amount is analyzed. The seal capacity computation method of multi stage seals is presented. The influence of rotating speed on the seal capacity is indicated. It has been found that the computation results agree well with the experimental ones.