The changes of surface composition of polyvinylchloride (PVC) films induced by X ray in ultrahigh vacuum are studied using X ray photoelectron spectroscopy(XPS). The results show: (1) The degradation of PVC occurs accompanied with the elimination of HCl and the formation of polyene sequence of longer length. (2) Selfacceleration exists in the degradation of PVC, and it is concerned in the power of irradiation. (3) During initial periods irradiated, the loss of chlorine is only concerned in time, and not in depth: after a time, the degradation is accelerated, the surface enrichment of Cl is observed because HCl can not be in time eliminated from the surface.(4) The longer the irradiation time, the more the loss of chlorine: the more the power of irradiation, the more the loss of chlorine.