The reservoir of the first member of Zhujiang Formation in Wenchang area of Pearl River Mouth Basin is characterized by high content of shale, and the difference of resistivity between oil layer and water layer is little. So it is difficult to identify the oil-bearing reservoir and water-bearing reservoir by well logging interpretation. Based on the analysis of reservoir lithology, imaging data of thin interbed and litho-electric experiment in various salinity conditions, this paper studied the impact of different factors on the formation resistivity. The results show that the main causes of low resistivity oil layer are fine lithology, high content of shale, high content of irreducible water saturation and middle formation water salinity. Based on the genesis of low resistivity, tri-water model was used to evaluate the water saturation of low resistivity oil layer in the study area. The calculation results are consistent with the sealed coring results.