With the continuous increase in China’s energy demand, submarine shallow gas, once considered a marine geological hazard, has gradually become an important target for hydrocarbon exploration and development. This study focuses on a shallow gas reservoir, the A Oilfield, in the Bohai Bay Basin, conducting laboratory testing of mechanical parameters on drilling core samples and analyzing their deformation characteristics. By integrating in- situ stress conditions and geological data, a coupled fluid flow–stress model was developed, and geomechanical simulations were performed under different depletion scenarios. The results indicate that the target reservoir exhibits low core strength, significant failure- induced deformation, and strong plastic characteristics. Based on the deformation characteristics, the mechanisms of tensile and compressive—shear stress zone formation during reservoir depletion were analyzed. Additionally, the evolution of the stress—strain field obtained from geomechanical simulations shows good consistency with analytical models. Further assessment of mechanical failure and subsidence risk suggests that the optimal depletion level of the target reservoir should not exceed 50%, aligning with the development extent of shallow gas reservoirs in offshore Myanmar. These findings provide valuable insights for developing offshore shallow gas reservoirs in China.