This study focused on the beach-bar sandstone reservoir in the upper part of the fourth member of Shahejie formation in the southern slope of Dongying sag. Core, thin sections, physical properties and other technical methods were used to analyze the characteristics and genesis of effective reservoirs. The results show that the reservoir pore space includes mainly primary pores with partial secondary and mixed intergranular pores. Favorable sedimentary environment is the foundation of high-quality reservoirs. Different kinds of diagenesis and their intensity affect the quality of reservoir. Hydrocarbon charging and overpressure are the favorable preservation conditions of reservoir properties, and their interactions affect the types of reservoir space, the distribution and combination, and evolutional characteristics of reservoir. The distributions of pore space in effective reservoirs are sectionalized in depth. At 1.0-2.0km, weak diagenesis and well preservation of primary porosity in ordinary pressure form effective reservoirs which contain mainly primary pores. At 2.0-3.1km, dissolution increases reservoir space. Hydrocarbon charging and overpressure save reservoir space and form effective reservoirs which have primary pores, secondary pores and mixed pores. At 3.1-3.6km, hydrocarbon charging and overpressure hold part of primary pores and form effective reservoirs which contain mainly primary pores and a few secondary ones.