Abstract: The characteristics of the low permeability reservoirs in the 5^th Member of Wenchang Formation of the Y Oilfield in the Pearl River Mouth Basin of the South China Sea and their controlling factors remain unclear up to date, and the geological basis for large scale oil development is insufficient. In term of these problems, various analysis and testing data, such as the data of cast thin section, scanning electronic microscope, and core physical properties, are used conducted in this study. It is revealed that lithic sandstone dominates the reservoir, followed by feldspathic quartz sandstone and feldspathic lithic sandstone. The sandstones are low in porosity and very low in permeability. The research on the main controlling factors on reservoir physical properties further suggests that the tectonic uplifting simultaneously with sedimentation may reduce the compaction of the reservoir to make its physical properties better. Depositional processes control the compositional and texture maturities and the distribution of microfacies, and finally affects the porosity and permeability of the reservoir directly, and the reservoir diagenesis indirectly. Quantitative study of reservoir diagenesis suggests that compaction plays a leading role in reservoir physical properties, followed by cementation and dissolution. The research has much improved the understanding of reservoir characteristics and physical property control factors for the 5^th Member of Wenchang Formation of the Y Oilfield in the Pearl River Mouth Basin, and will provide a solid geological basis for the scale development of the oilfield.