Photocatalytic conversion of polylactic acid (PLA) into high-value-added pyruvic acid (PA) presents a promising strategy for carbon-efficient resource cycling. However, successive dehydrogenation at the α-C of lactic acid (the monomer of PLA) remains a critical challenge. In this study, we fabricated an Au-modified CdS nanosphere photocatalyst (Au-CdS), where the introduction of Au facilitates the activation of the Cα-H bond in lactic acid (LA). Through combined theoretical calculations and in-situ EPR analysis, Au-CdS reduces the Cα-H bond cleavage energy barrier from 1.02 eV to 0.49 eV, substantially elevating the generation of carbon-centered radical intermediates. In addition, Au-CdS demonstrates enhanced separation efficiency of photogenerated electron-hole pairs relative to pristine CdS, thus efficiently activating O2 to generate O2•− radicals and further driving the conversion of O2•− to •OH radicals. The produced •OH radicals facilitate the successive dehydrogenation reactions via interacting with the Cα-H and α-OH bonds on the α-C of LA, thereby promoting the selective conversion of LA to PA. As a result, the Au-CdS catalyst achieves a PA production rate of 1340 ± 25 μmol g-1 h-1, surpassing that of pristine CdS by 8.9-fold. This work presents a novel strategy for the value-added upcycling of plastic waste.