Abstract: The global plastic waste crisis is worsening, and effective chemical upcycling technologies are urgently needed. Upgrading waste polyolefins into valuable liquid fuels under mild conditions is a promising route for plastic recycling. However, the complex reaction networks involved make it difficult to understand the underlying mechanisms and optimize catalysts. Here, offline gas chromatography-mass spectrometry (GC-MS) was combined with synchrotron radiation photoionization mass spectrometry (SR-PIMS) to investigate the catalytic cracking of high-density polyethylene (HDPE) over Pt/ZSM-5 at 280 ℃. Pt/ZSM-5 catalysts with different loadings were prepared by impregnation and characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), N₂ physisorption, and ammonia temperature-programmed desorption (NH₃-TPD). TEM results showed the uniform dispersion of ~4.7 nm Pt nanoparticles at a loading of 0.5 wt%, whereas higher loading (1.5 wt%) led to agglomeration and partial pore blockage. This observation correlated with a volcano-shaped trend between Pt loading and catalytic performance. NH₃-TPD confirmed that Pt did not significantly alter the acid site distribution of ZSM-5, indicating that the observed catalytic effects are metal-specific. Offline GC-MS analysis of the liquid products enabled precise quantification of product distributions, identifying 0.5% Pt/ZSM-5 as the optimal catalyst. Compared with HZSM-5, Pt increased the selectivity of liquid products by about 50% and the aromatic yield by 33%. SR-PIMS, with its tunable photon energy, provided deeper mechanistic insights. Under low-pressure conditions (266 Pa) using 9.2 eV photons, reactive C₄-C₉ alkyl radicals were captured in situ for the first time during HDPE catalytic cracking. A direct comparison between HZSM-5 and 0.5% Pt/ZSM-5 revealed that Pt significantly reduced radical concentrations, offering direct evidence for radical quenching by Pt sites. Under near-ambient-pressure conditions (5×10⁴ Pa) with 11 eV photons, real-time monitoring of product evolution during temperature-programmed reactions elucidated the temperature-dependent interplay between cracking, hydrogenolysis, and aromatization. Pt modification notably enhanced medium-chain olefins (C₅-C₈) and aromatics at higher temperatures (>300 ℃), while light alkanes showed distinct evolution patterns that reflected shifts in the dominant reaction mechanisms. Furthermore, the detection of H₂ at 15.5 eV confirmed Pt’s low-temperature dehydrogenation activity at 280 ℃. Based on this complementary mass spectrometric evidence, a reaction mechanism involving three primary Pt-mediated pathways was proposed: 1) hydrogenolysis of C-C bonds to enhance alkane production; 2) dehydrogenation to lower the energy barrier for aromatization; 3) radical quenching, in which reactive intermediates are converted to stable products via surface alkyl species. The synergy between Pt metal sites and ZSM-5 acid sites thus enables selective liquid hydrocarbon production under mild conditions. While this work establishes a complete mass spectrometric evidence chain for metal-acid bifunctional catalysis in polyolefin upgrading, several aspects warrant further investigation. The exact nature of Pt-radical interactions and the dynamics of hydrogen transfer between metal and acid sites remain to be elucidated, potentially through advanced spectroscopic techniques and theoretical calculations. Moreover, extending this methodology to real-world plastic waste containing additives and contaminants would help assess its practical applicability. The demonstrated capability of SR-PIMS for capturing reactive intermediates opens new avenues for mechanistic studies in polymer catalysis.