Hangjinqi area of Ordos Basin is a key research area for tight gas exploration，and the Lower Shihezi Formation is mainly lithic sandstone which is a typical low-porosity and extra-low-permeability reservoir. According to the analysis results of cast thin section observation，cathodoluminescence，SEM，capillary pressure test，X-diffraction of clay minerals and porosity and permeability，the main types of diagenesis and its characteristics of the tight sandstone reservoir in the Lower Shihezi Formation in Hangjinqi area of Ordos Basin were studied. And the evolution sequence of diagenesis was established and its effects on pore development were analyzed. A quantitative model for porosity evolution was constructed.The results show that the tight sandstone reservoir with secondary pores and microfracture is the main storage space in the Lower Shihezi Formation of the study area. There are various types of diagenesis in the Lower Shihezi Formation，which mainly include compaction，cementation，metasomatic alteration and dissolution. The compaction is the main factor causing great loss of primary pores，and the decrease of porosity is about 20.25%；the cementation destroys the physical properties of the reservoir. The early and the late cementation result in a decrease in porosity of about 5.58% and 6.06%，respectively.The secondary pores formed by dissolution leads to an increase in porosity of about 5.20%.