Abstract: Aiming at reconstruction of paleobathymetric depth in the deep-water sedimentary basins in the northern South China Sea, this paper takes the Baiyun Sag of the Pearl River Mouth Basin as an example. According to the sedimentary characteristics of the continental shelf and slope, this study is based on depositional architecture by characterization of seismic profile and bedding flattening in third-order sequence in five critical geological stages（21, 19.1, 13.8, 10 and 5.33 Ma） of post-rift stage in Baiyun Sag. The results show that the average paleobathymetric depth in the 21 Ma and 19.1 Ma stages is less than 300 m, from then to 13.8 Ma, the maximum paleobathymetric depth is 900 m. During the 10 Ma and 5.33 Ma stages, the paleobathymetric depth expanded from 1100 m to 1600 m. From 21 Ma to 5.33 Ma, the shelf break belt had spreaded along the northwest-southeast direction on the northern slope of Baiyun Sag. The paleobathymetric depth of northwest shelf area is shallow, and the southeast continental slope and deep sea plain area is deep on the plane. The paleobathymetric depth of the Baiyun Sag had undergone a continuous deepening process of shallow sea to semi-deep sea and finally deep sea, and there were two stages (13.8 Ma and 5.33 Ma) much faster paleobathymetric depth rate than other stage in Baiyun Sag, the paleo-water depth change rate is 117 m/Ma and 103 m/Ma respectively,and the deepening rate of the paleobathymetric depth was the largest in the 13.8 Ma stage. The result of paleobathymetric has important research and guiding significance for the determination of the key parameters of paleobathymetric for thermal depression history research and the restoration of sedimentary paleomorphology.