Abstract: Based on the concept of sequence stratigraphic meth- odologies,the cyclic sequence in Lower Cretaceous in Bei- er Sag is classified and correlated in detail,the strata con- trast framework of cyclic sequence is defined and estab- lished,and the characteristic of different grade sequence boundary is analyzed.The study of the sedimentary sys- tem shows the spatial distributions and sedimentary as- semblages of depositional system vary significantly in dif- ferent depositional stage.The sequence of SQt-SQn2 formed in faulted down lacustrine basins,which developed the sedimentary assemblage as alluvial fan,fan delta, sublacustrine fan and Shore-shallow lacustrine.The se- quence of SQd1-SQd2 formed in semi-deep lake and deep lake in depressed stage,which developed the sedi- mentary assemblage as delta,shore-shallow lacustrine, shallow lacustrine and semi-deep lake.In different sedi- mental stage,the distribution of different sub-structural units control the special distribution of sedimentary faci- es,and the architecture of sedimentary facies of system tracts vary.The central sub-sag depression of western Beier Sag,the antithetic contemporaneous fault and downdip areas of palaeoslope break in western Beier sub- sag,and southern Beier sub-sag,where are the main ob- jects of exploration.