Abstract: The strong lateral and vertical velocity variations in complex structures pose significant challenges to velocity model building and seismic imaging in marine exploration. To address these challenges, this study investigates the application of full-waveform inversion (FWI) driven by wide-azimuth long-offset seismic data. We examine the advantages of FWI over ray-based tomography in resolving complex geological structures, and also discuss some key factors in the 3D FWI algorithm that are critical for improving computational efficiency without compromising imaging quality. We present a case study from Bohai Oilfield, which utilizes wide-azimuth long-offset ocean-bottom-cable data for a systematic analysis involving data preparation, implementation of multi-scale inversion strategies, and quality control. The results demonstrate that the high-precision FWI velocity model allows direct interpretation of major structures like fractures, igneous rocks, and gas clouds, and also yields depth migration results with substantially improved imaging quality over those from ray tomography-derived velocity models.