Abstract: Shallow gas accumulations in Hangzhou Bay, predominantly composed of methane (CH₄), represent a significant geological hazard with implications for marine engineering safety and regional carbon cycling. This study employs a multidisciplinary approach to investigate the composition, origin, and migration mechanisms of shallow gas through a comprehensive analysis of Core Y2 sediments, including gas composition, stable isotopes (δ¹³C-CH₄ and δ¹³C-CO₂), grain size distribution, porosity, and water content. Results demonstrate that the shallow gas is primarily biogenic, with δ¹³C-CH₄ values ranging from −94.1‰ to −57.4‰, consistent with microbial CO₂ reduction in saline marine environments. Elevated CH₄ concentrations (up to 116940×10⁻⁶) correlate with distinct sedimentological facies characterized by balanced proportions of sand, silt, and clay, which facilitate gas accumulation. The study elucidates the role of Quaternary sea-level fluctuations and sedimentary dynamics in creating heterogeneous gas reservoirs, with migration governed by sediment permeability and capillary effects. These findings provide critical insights for assessing shallow gas hazards and optimizing engineering projects in Hangzhou Bay.