The onshore‐offshore deep seismic experiment was carried out for the first time and filled in the gap of the seismic surveys in the transition area between South China and northeastern South China Sea. The seismic data were analyzed and processed. The different seismic phases were identified and their travel time arrivals were modeled by the ray‐tracing program MacRay to study the P‐wave velocity crustal structure. The result shows that the crustal structure of this area has the feature of the continental crust. The crustal thickness is gradually decreasing southward along the onshore‐offshore seismic line. A low‐velocity layer (5.5‐5.9km·s−1) of 2.5‐4.0km thickness exists generally in the middle crust (at about 10.0‐18.0km depth), and is also thinning seaward. No obvious high‐velocity layer appears in the lower crust. The Binhai (littoral) fault zone in this crustal structure is revealed as a low velocity zone, which is located about 35.0km southeast to the Nan'ao station and corresponding to the gradient belts of gravity and magnetic anomalies. The depth of the Binhai fault zone extends down close to the Moho discontinuity, which is the boundary between the normal continental crust of South China and the thinning continental crust of the sea area.