We examined surface damage for polyvinyl chloride (PVC) sample after 260 minutes at an unmonochromated Al Kα X-ray source power at 600 W, because PVC decomposes very quickly due to the X-ray irradiation. The pendant Cl groups of the PVC were estimated to cleave more rapidly than C-C bonds in the main chains from two-center bond energies calculated by AM1 method using the model pentamer. The valence XPS of the polymer at the initial time, and after 120 and 260 minutes of X-ray irradiation were analyzed by deMon density-functional theory (DFT) calculations using the model pentamer. The simulated results afforded us that the pendant Cl groups of PVC cleave mainly due to the X-ray radiation.