Magnetic fluid was invested in 20thcn 60s, and nowadays, it is used as an effective way for mechanical seal. This paper follows the seal requirements of stirred reactor rotating axis, analyzes the applicability for its seal condition. Meanwhile, in this paper the magnetic field is calculated by numerical simulation, the free boundary of magnetic fluid is analyzed in conditions of static seal and dynamic seal. The boundary analysis for magnetic fluid seal can directly show its position, when the boundary of magnetic fluid moves to the lower extreme of pole teeth, it can be considered the seal is invalid. This is a simple way to supervise the magnetic fluid seal.