Abstract: In this paper, the concave hem isphericalw ake shield facility ( C H W S F) isstudied w hich is in the state of steady, non uniform w ith the closed sphericaltheoreticalm odel. The distribution ofthe m olecularnum berdensity ofthe experim entalzone is calcu lated and the m ain effect factors are discussed. The result show s thatthe C H W S Fcan ob tain extrem e high vacuum levelw hose m olecular num ber density is about 108m - 3,the m aincontribution to the m olecular num ber density com es from the outgas ofthe C H W S F, andthe increasem ent of the coherent coefficient of the C H W S F can't effectively im prove thevacuum levelon occasion of absence ofgasload.