Abstract: Kaolinite is one of the commonest authigenic clay minerals in sandstone reservoirs from the point of view of reservoir evolution. Kaolinite generally existed in pore-throat and is an indicator mineral of dissolution of feldspars and development of secondary pores. Based on thin-section identification, scanning electron microscope analysis, X-ray diffractometric measurement, and study of the distribution and forming mechanism of authigenic kaolinite which influenced quality of Paleogene and Neogene reservoirs in Xihu Sag, it was indicated that the content and distribution of authigenic kaolinite was related to the dissolution of aluminosilicate minerals like feldspars which dissolved by organic acid and meteoric water in the diagenesis process and the content of kaolinite was well correlated to reservoir physical properties. Content of kaolinite in Pinghu Formation of the west slope was obviously higher with better development conditions for diagenesis and secondary pores than that of center inversion structure belt. Content of kaolinite in the southern area of Xihu Sag was relatively higher with better development conditions for secondary pores and better reservoir quality than that of northern area.