Abstract: On the basis of drilling core observation, the diagenesis and porosity evolution characteristics of the Chang 8 reservoirin Qincheng-Heshuiarea have been comprehensively studied through using ordinary thin section of sandstone, casting thin sections, scanning electron microscope, clay minerals analysis and other methods. The results show that: ①the Chang 8 reservoir in Qingcheng-Heshui area is mainly a set of sandstones with delta front facies, which are composed by lithic arkose sandstone and a small amount of feldspar sandstone; ②the sandstones of the Chang 8 reservoir have mainly experienced some diagenesis, such as compaction, cementation and later dissolution; ③the diagenesis stage has reached the the stage of A period, which can be reflected by homogenization temperatures of inclusions, clay minerals content combination, cement, pore types, and other indicators reaction; ④Early compaction is the main factor for the decrease of pore space, with the average loss rate of 21.58%, and the average porosity loss rate caused by cementation is 6.89%. Butthe later dissolution is an important factor for improving the sandstone reservoir, which resulted in an average porosity incrasing rate of 3.97%. The research on the pore evolution characteristics and diagenesis stages of Chang 8 sandstone reservoir in Qingcheng-Heshui area plays an importantrole for the development of low permeability tight reservoirs.