Abstract: In this paper , the seal capacity computation and experimental study for a high pressure differential magnetic fluid seal was carried out based on the theory of the magnetic fluid seals . The method for computing the seal pressure differential of both single-stage and multi-stage seals was presented. The seal capacity was analysed quantitatively through numerical computation on the magnetic field. It was pointed out that the seal capacity can be increased greatly by combining several multi-stage seals into one seal unit, forming a multi-pole multi-stage seal. But the capacity can not be increased infinitely. Instead it tends to a limited value. Because the magnetic fluid under each stage is not equal in amount when a pressure is applied on the seal, there is a difference between the practical seal capacity and its computed value. The difference increases with the increasing of the pole number. It is believed that if the magnetic fluid can be supplemented into the sealing gap while appling the pressure differential on the seal device, the difference can be reduced and the seal capacity can be increased further. The seal pressure differential of the seal device designed in this paper reaches 1. 4MPa. The reasions for the difference between the practical and computational values of the seal capacity were discussed at the end of the paper.