Abstract: Based on the data of cast thin section, scanning electron microscopy(SEM), X-ray diffraction and physical property analysis, the paper studies the features of petrology and diagenetic process and their influences on reservoir pore evolution of Chang 61 tight sandstone reservoir in Jiyuan area, Ordos basin. The results show that the tight sandstone reservoir in the study area experienced a complicated diagenetic process and most of the reservoir has entered phase A of the middle diagenetic stage and the rest has been in the early phase B of the middle diagenetic stage. Based on the establishment of quantitative porosity evolution model, it is found that compaction is the main factor causing the reservoir densification which results in the decrease of the reservoir porosity by 21.52% with the porosity loss rate of 55.72%; cementation damage on reservoir is the secondary with the porosity loss rate of 32.41%; dissolution can increase the porosity by the average of 4.36%. Using burial history to match comprehensive porosity evolution history, the paper characterizes the influences of burial depth and diagenesis on porosity quantitatively. The analysis on 4 typical samples further indicates that the Chang 61 tight sandstone reservoir in the study area belongs to compaction-type diagenesis, and compaction and cementation are the key factors causing reservoir densification