The ferrofluid seal is a rotary dynamic seal widely used in the chemical industry, shipbuilding, aerospace, aviation, and other fields, with many advantages, such as high reliability, low wear, and low leakage rate. The direct sealing reason is that the ferrofluid ring isolates the sealed gas from the atmosphere. Hence, the boundary deformation and fluctuation are significant for the ferrofluid seal design. However, the ferrofluid boundary is wrapped in the shell and pole shoe, which is challenging to observe directly. This paper presents an experimental ferrofluid rotary seal device, which can directly monitor the boundary changes under different rotation speeds and pressure differences. Combined with the simulation result, the boundary fluctuation caused by rotation affects the pressure transfer process and reduces the sealing performance. The ferrofluid boundary obtained by the experiment is essential in thoroughly investigating the rotary seal failure mechanism to improve the ferrofluid rotary seal.