Abstract: The northeastern continental margin of the South China Sea represents the region of initial rifting in the South China Sea Basin. To understand the deep crustal structure and tectonic evolution processes in this region, a deep seismic profile, DP12B, was deployed traversing the Southwest Taiwan Basin along the NW-SE direction. This study primarily outlines the preliminary data processing procedures for the DP12B profile, encompassing navigation data processing, trimming of raw OBS data, and OBS position and time corrections. Then, phase identification was conducted, and the accuracy of phase identification was assessed using Rayinvr software for ray tracing and travel time fitting. The results indicate that the OBS position and time corrections were effectively applied. The comprehensive seismic profile derived from CRG (Common Receiver Gather) successfully identified multiple clear P-wave phases, including Ps, PsP, Pg, PcP, PmP, and Pn. The furthest phase could be consistently traced up to 90 km away. The morphological features and fitting of seismic phases at stations DP12B-9 and DP12B-7 reveal a pronounced crustal thinning to 3 km in the southern depression of the Southwest Taiwan Basin, possibly accompanied by mantle serpentinization. The overall data quality of the DP12B profile is excellent, providing a robust basis for velocity modeling and offering reliable support for further exploration of the geological structure and evolutionary processes in the South China Sea region.