A vibrated-bed reactor with an online UV-Vis measuring system was established in order to reveal the dynamic process of UV-enhanced chlorination of polyvinyl chloride (PVC). Influences of temperature, radiation time, and interval time of intermittent initiation on chlorination process as well as the overlapping enhanced effect of adjacent UV radiations were analyzed. By properly controlling the intermittent UV radiation, one can obtain a considerable steady reaction rate during the chlorination process to avoid the temperature runaway due to over-accelerated reaction. Solid-state nuclear magnetic resonance identified the microstructure change of chlorinated PVC particles. The whiteness value after thermal aging was determined to uncover the relationship between thermal stability and double-bond content, where the results accorded with the related theory on polymer dehydrochlorination.