Based on a large amount of analysis and statistics for the well logging data in South China Sea, the authors believed that the oil layer and the water layer in one sequence with the same porosity and under the same facies and correspond to different wave impelence, which can be used as oil and water identification mark for reef-beach facies. The research idea in this paper can be expressed as below: at first through well logging analysis, porosity inversion and wave impedance inversion are performed, then the wave impedance difference between oil layer and water layer under the same facies condition was utilized to conduct oil and water identification. The application example shows that applying of the method could clearly delineate the distribution of the oil reservoir and water zone.