Huizhou Depression is an important oil and gas-rich region of Pearl River Mouth Basin. HZ25-7 structural belt is an important area designed for seeking oil and gas of Wenchang Formation in Huizhou Depression. The research of reservoir characteristics and mechanism of Wenchang Formation is the foundation of oil exploration breakthrough. Based on comprehensive analysis of various data such as drilling, casting thin sections, scanning electronic microscope, physical properties analysis and mercury penetration test, this paper analyzed the reservoir characteristics and mechanism of Wenchang Formation of HZ25-7 structural belt. The results show that sandstones are mostly litharenite and feldspathic litharenite, and are generally immature both texturally and compositionally. The average porosity and permeability of the sandstones are 12% and 7.1 mD respectively, and it is emphatically defined as the low porosity and low permeability reservoir. Pore types are mainly intragranular dissolved pores and primary intergranular pores, and its micro-pore structure is typical medium-micro pore and fine throat, with poor pore connectivity. Such low porosity and permeability reservoir of Wenchang Formation is controlled by depositional conditions and diagenesis. Braided fluvial deltaic depositional environment characterized by near provenance determined the large quantity of ductile grains in detrital composition and the infilling of part of primary pores by resulting matrix. Strong compaction results in primary porosity loss significantly. Due to low content in soluble grains, the improvement of physical properties mediated by organic acid is not obvious. Clay and carbonate cementations lead to continued worsening of reservoir properties and tightening, but abnormal overpressure is constructive to the preservation of reservoir porosity.