Alkali diagenesis has been discovered firstly in Xujiahe formation in Sichuan basin. Alkali diagenesis is characterized by quartz dissolution pore. Based on thin section identification, scanning electron microscope and X-ray diffraction analysis, and considering the geological background, the characteristics and formation mechanism of alkali diagenesis in Xujiahe formation are studied thoroughly. The results indicate that the sandstone reservoir in Xujiahe formation has experienced a strong diagenesis and entered the stage of late diagenetic B. Diagenetic environment experienced a transition from acidity to alkalinity. Secondary pores produced by quartz dissolution contribute for the better quality of Xu_2 reservoir.