Abstract: The A gas field lies in the Xihu Sag of East China Sea Basin, with water depths of 90–110 m. Its main gas layers are buried over 4,000 m below sea level. Current prestack depth migration (PSDM) seismic data struggles to detect small faults, map detailed reservoir features, and predict drilling depths accurately. Reprocessing the seismic data is critical to improve image quality and support gas field development. Based on raw seismic data, the reprocessing workflow prioritized noise suppression in marine environments, source-receiver ghost wave attenuation, and shallow-water multiple wave suppression. Key steps of seismic signal processing included: high-precision velocity analysis, curved-ray Kirchhoff prestack time migration, globally optimized grid tomography velocity inversion, well-seismic joint anisotropic velocity modeling, and Kirchhoff anisotropic prestack depth migration. The reprocessed data shows clearer images with less background noise, better-defined faults, and more accurate reservoir details in deep zones and drilling depth predictions. This workflow can effectively enhance seismic data for deep and low-permeability gas fields, and thus can be adapted to similar offshore projects.