Abstract: Based on the analysis of major element characteristics of 161 surface sediment samples from the Caofeidian offshore area of the Bohai Sea and 30 surface sediment samples from major rivers around Bohai Bay, the assemblage characteristics and influencing factors of major elements were studied. Considering the characteristics of major elements in stream sediment along Bohai Bay, the weathering and provenance were discussed using the chemical index of alteration (CIA). Results show that SiO₂ and Al₂O₃ constitute the predominant elements in the surface sediment of the study area. Seven elements, i.e., Si, Al, Fe, K, Mg, P, and Ti, clearly demonstrate a significant "grain size effect". Except for Si, the distribution patterns of other six elements follow a discernible trend consistently. Higher concentrations were observed in the southern and western regions, while lower concentrations prevail in the north and east. It is worth noting that CaO and Na₂O exhibit a substantial influence from their provenance, while the distribution of MnO appears to be closely correlated with redox conditions. The CIA values of sand and silt-sand sediments in Caofeidian sea area are 48.4 and 57.2, respectively. This index is positively correlated with sediment particle size, and it is in the primary weathering stage with the surrounding stream sediments. Furthermore, the source rock composition of sediments in the Caofeidian offshore area bears striking similarities to that of adamellite. Topographic and climatic variations are important factors affecting the differences in the weathering intensity of sediments in the Luanhe River system and the Haihe River system.