Abstract: The eustatic sealevel change rule has been studied by using element geochemistry feature and seismic geometric configuration feature in the Cambrian-Ordovician in Tarim Basin, and the comparative sealevel change curve has been obtained, which can be divided into three firstgrade cycles, eight secondgrade cycles, seventeen thirdgrade cycles and more high frequency concussion cycles in the Cambrian-Ordovician in Tarim Basin. There occurred the secondgrade eustatic sealevel change cycle three times during Cambrian and two times during the early Ordovician, and the extent of the sealevel rise in the early Ordovician is higher than that in Cambrian. In the middlelate Ordovician there occurred the secondgrade eustatic sealevel change cycle three times, and the first time occurred at low system tract of sea level with small rise extent of the sealevel, but the sealevel rapidly reached the highest water level state in the later two times. The eustatic sealevel change controls on the basin sediment infilling and sequence formation evolution. The cyclicity of the sealevel changes are corresponded with the cyclicity of the basin sedimentation, and are represented by the sedimentary facies evolutions.

Key words: Tarim Basin, sequence stack, high frequency cycle, Cambrian-Ordovician, sealevel change