This paper is the result of coastal geological survey engineering. Objective The second marine layer in the coastal lowland of Bohai Bay remains controversial, which restricts the progress of sea level change research in this area. This study performs optically stimulated luminescence dating on this layer, expecting to calibrate the precise stratigraphic age and the corresponding marine transgression episodes. Methods By taking the CZ80, CZ85, and CZ66 boreholes on the west coast of the Bohai Bay as the research targets, based on the examination of sedimentary structures, textures and foraminifera abundance in the cores and sedimentary microfacies, the second marine layer was identified, and a comparative profile of the second marine layer was constructed. The formation age of this layer was precisely determined through the optically stimulated luminescence dating method, and the relationship between the development duration of the second marine layer and global sea level changes was further explored. Results The CZ80, CZ85, and CZ66 boreholes all intersected the second marine layer beneath the Holocene stratum (including the first marine layer). The base of the marine layer was embedded at a depth ranging between 35 and 33 meters, corresponding to the elevation ranging from –31.13 to –28.39 meters in the Yellow Sea. The top of the marine layer was buried within a depth range of 32 to 20 meters, with an elevation varying from –25.58 to –15.39 meters. The optically stimulated luminescence dating outcomes reveal that the occurrence time of the marine transgression event corresponding to the second marine layer was approximately 94 to 71 ka. Conclusions The optically stimulated luminescence (OSL) dating outcomes reveal that the second marine layer in this region commenced at 94 ka or earlier, and the influence of seawater terminated approximately 71 ka ago. Through comparison with the global sea–level curve, the spatio–temporal attributes of the second marine layer are consistent with the MIS5a stage, chronicling the marine transgression event during the MIS5a stage.