Abstract: The third member to lower submember of the second member in Shahejie Formation of Dongying Sag can be divided into four third-order sequences on the basis of seismic date, lithology and electric characteristics, etc. Two middle base-level cycles(third-order sequences) are distinguished in the strata studied, which are the middle submember of the third member of Shahejie Formation, middle base-level dropping half-cycles, and the upper submember of third member to the lower submember of the second member of Shahejie Formation middle base-level raising half-cycles. Three sequence boundaries among T4, T6 and SB are recognized in the strata studied. T4 and T6 are seismic symbol-layers, which are in response to the maximum flood surfaces. The development characteristics of the middle base-level cycles control the distribution of reservoir sand bodies and the delta front sands bodies are developed in the middle of the basin bottom fans and slop fans. Sand bodies are developed in the early of middle base-level raising half-cycles. They are favorable for oil bearing, so that the sand bodies are prospected districts for subtler reservoirs.