Huizhou depression is one of the most prolific hydrocarbon areas in Pearl River Mouth basin. Plentiful Neogene oil and gas have been found, but no breakthrough has been made in Paleogene sediments. Reservoir porosity and permeability are thought very low in Paleogene, and this inhibits the exploration progress. As to the deep reservoir, the factor of porosity is in connection with petrographic composition and architecture. The skeleton components and matrix components of the sandstone are basic substances participating in diagenesis. Feldspar sandstone, feldspathic quartz sandstone and lithic arkose are main rock types in Zhuhai and Enping Formations, which are characterized by coarse grains, poor grading, subangularity, low compositional maturity, and moderate to bad textural maturity. The storage capability is considerably impacted by diagenesis and porosity evolution. Based on slices observation and cores analysis and using such means as scanning electron microscope (SEM), X-diffraction, cathodoluminescence and organic acid test, the authors studied diagenesis and porosity evolution. The results show that the sandstone porosity is decreased mainly by compaction and cementation and subordinately by quartz overgrowth and authigenic mineral filling. Abundant secondary pores are produced by the denudation of such unstable components as feldspar, matrix and cuttings, and this leads to the increase of the percentage of voids.Researches reveal that the deep reservoir has experienced four diagenesis evolution phases, i.e., syngenetic stage, eogenetic stage (composed of A-stage and B-stage), anadiag enetic stage (comprizing A-stage & B-stage) and late diagenetic stage. Well data show that most of the reservoirs have attained the eogenetic stage,whereas segmental sandstone (under 4 000 m) has entered into the late diagenetic stage. Primary porosity was decreased quickly by compaction at the eogenetic stage (A→B) because of the increasing overlying load. Linear contact occurred at the B-eogenetic stage. Hydrothermal fluid of organic acid was discharged to fastigium with most kerogen being transformed into hydrocarbon at the anadiagenetic stage (A→B). Due to increasing acidity, the unstable components of aluminosilicate framework ingredients such as feldspar, matrix and cuttings were dissolved, so a lot of secondary pores were generated, and the physical property of reservoir was improved. At the late diagenetic stage, organic material was changed from mature to highly mature, organic acid concentration was decreased to the lowest and destroyed by thermal activity, stratigraphic water was gradually turned into alkaline, authigenic minerals were developed generally, and the reservoir physical property was changed to bad by the cementation of anhydrite. Just because lots of secondary pores were generated at the eogenetic stage, it improved the physical property and increased the potential of deep reservoir exploration.