Abstract: By means of rock (casting) thin-section identification, scanning electron microscope and X-ray diffraction, a detailed study was carried out on the characteristics and genetic mechanism of quartz dissolution in tight sand reservoirs of Xujiahe Formation in transitional zone of the Central and Southern Sichuan Basin as well as its impacts on pore development. Research results have indicated that massive quartz dissolution existed along grain edges and secondary enlarged edges, as well as in the interior of grains. Quartz dissolution mainly took place in subage A₂ of the middle diagenetic stage. There are two genetic mechanisms of quartz dissolution. One is the direct dissolution in alkali diagenetic environment with weak dissolution degree. The other is the metasomatic dissolution by clay minerals and carbonate minerals, of which the metasomatic dissolution of clay minerals against quartz grains is more intense. After the dissolution of quartz grains, a large number of secondary pores are formed to effectively increase reservoir spaces and improve pore structure as well as reservoir porosity and permeability. In particular, the secondary pores that were generated by metasomatic dissolution of clay minerals against quartz made considerable contributions to reservoir pores.