Abstract: In order to clarify the characteristics of trace elements and rare earth elements (REEs) in the Upper Paleo- zoic mudstones of the Bohai Bay basin and their geological significance,combined with outcrop,core and thin section observation,whole rock X-ray diffraction and clay mineral X-ray diffraction analysis,the trace elements and REEs in 36 mudstone samples from Bohai Bay basin were determined by inductively couple-plasma mass spectrometry ( ICP- MS). The development characteristics and composition,geochemical parameters and spatial distribution of trace ele- ments and REEs of Upper Paleozoic mudstones,paleo-salinity,the characteristics of provenance,deposition rate and the significance of sequence stratigraphy were discussed. The results show that the contents of trace elements in the Upper Paleozoic mudstones in Bohai Bay basin are similar to that of the upper crust,the REEs are relatively enriched and the differences between light and heavy REEs are obvious. The variegated mudstones have undergone the rewor- king of weathering and leaching and are composed of bauxite or bauxitic mudstone,the activation and migration of trace elements and REEs result in outliers in the parameters,therefore,the element information during deposition has been destroyed. From the Benxi formation to the Shanxi formation,the REEs contents in mudstone continuously in- crease. The REEs contents of the Lower Shihezi formation are lower than that of the Shanxi formation,and from the Lower Shihezi formation to the Kuishan member of the Upper Shishanzi formation,the REEs contents gradually in- crease. The Sr / Ba values reflect the process of transgression from development to decline and intermittent marine- flooding in Late Paleozoic. The tectonic setting of provenance area has the characteristics of active continental margin, the lithologies are dominated by sedimentary rocks,granite and alkaline basalt,and the stratigraphic ages have the characteristic of getting old first and then getting new. The deposition rate increases gradually from the Benxi formation to the Kuishan member of the Upper Shihezi formation. The contents of trace elements Sc,Co,Ni,Rb, Sr, Ba and ∑REE in variegated mudstones at the top of the Lower Shihezi formation show abnormally low values,reflecting the large-area deposition intermittent effect,and the top surface of variegated mudstone bed could be used as sequence in- terface.