Degradable vinyl polymers by radical ring-opening polymerization have become a promising solution to the challenges caused by the widespread use of non-degradable vinyl plastics. However, achieving even distribution of labile functional groups in the backbone of degradable vinyl polymers remains challenging. Herein, we report a photocatalytic approach to truly random degradable vinyl copolymers with tunable main-chain composition via radical ring-opening cascade copolymerization (rROCCP). The rROCCP of the macrocyclic allylic sulfone and acrylates or acrylamides mediated by visible light at ambient temperature achieved near-unity reactivity ratios of both comonomers over the entire range of the comonomer compositions and afforded truly random vinyl copolymers with degradable units evenly distributed in the polymer backbone. Experimental and computational evidence revealed an unusual reversible inhibition of chain propagation by in situ generated sulfur dioxide, which was successfully overcome by reducing the solubility of sulfur dioxide in the reaction mixture. This study provided a powerful approach to truly random degradable vinyl copolymers with tunable main-chain labile functionalities and comparable thermal and mechanical properties to traditional non-degradable vinyl polymers.