The technique of large-sized rotary dynamic sealing is a key one for electron beam welding in partial vacuum ambient. The technical scheme of rotary dynamic seal and selection of its facility are discussed, and a set of experimental rig for testing large-sized rotary dynamic seal is developed. It is resulted from the test under the condition of 6×10~(-3) Pa vacuum and 3.8 r/min rotative velocity that (1) for this sealing facility, the major air leakage is due to the rotation instead of the porosity of materials or static leakage, which are secondary; (2) necessary vacuum can be guaranteed with a single-layer sealing mechanism without considering the double-layer one with sandwich; (3) the supporting function of rolling balls in this kind of sealing is very significant for their ability to reduce friction extensively while, at the same time, guarantee the airtightness. The experimental results show that the technical scheme of this kind of sealing is proper and this large-sized dynamic sealing facility designed is feasible.