Abstract: The first hard soil layer（FHSL） is widely distributed in the two wings of the Yangtze River estuary, and the study of its formation mechanism and engineering geological characteristics is of good guidance for engineering construction. Based on the survey data (935 boreholes with a total depth of 42128 m) and experimental data, the distribution boundary of the FHSL in the northern flank of the Yangtze River estuary was accurately confirmed for the first time, and the formation age, grain size characteristics, geochemical characteristics and engineering geological properties of the FHSL were studied. The study shows that the formation age of the FHSL was about 20 ~ 11 ka B.P. (OSL and ¹⁴C dating data). The water content of the FHSL tends to increase with depth, indicating that the climate was gradually cooler and drier from bottom to top. The particle size gradation, frequency curve of particle size distribution and C-M sedimentation diagram of the FHSL show that the FHSL mainly consists of fine sand, very fine sand and clay. The grain size frequency curves of the FHSL mainly show a single peak distribution, reflecting that the transport camp force was single before the material deposition. The deposition of soil particles were absolutely dominated by uniform suspension, and the deposition environment was a relatively stable low-energy environment. The development of the FHSL was controlled by climate and can be roughly divided into three stages: the first stage (about 20 ~ 15 ka B.P.) was the period of alternate deposition and soil formation, and the deposition was the main effect, and the thickness of FHSL was mainly controlled by this stage until the end of the last bloom ice period. The second stage (about 15 ~ 11 ka B.P.) was the period of exposed soil formation, at which time floods cannot form transgressive deposits, accretion basically stops, the thickness of FHSL no longer obviously increased. The formed area of the FHSL was erosion cutting by frequently shifting divergent river networks, forming many irregular ancient river channels and terraces. The FHSL gradually dehydrated into land and underwent the process of weathering and pedogenesis. The third stage (about 11 ka B.P. to present) was the inundation period. With the arrival of the Holocene, the climate warmed, and the sea level kept rising. The FHSL was buried by its overlying marine sedimentary layer, and the rock formation began until the present day. The soils have a high content of soluble salts and are typical chlorosaline soils. The soluble salt content has a trend of increasing from low to high, which was caused by the diagenesis process after the marine layer was covered with the FHSL.