This research took Lower Shihezi Reservoir in Yan’an, Ordos Basin as research object. Based on the achievement of former scholars, the article first analyzed burial history and palaeogeothermal history. Furthermore, it analyzed the diagenesis characteristics and divided the diagenetic stages through the application of test data, such as thin section, SEM, cathode luminescence, x-ray diffraction, homogenization temperature of fluid inclusion, etc. Finally, the article analyzed and identified the initial porosity model and various kinds of porosity evolution model in the process of diagenetic, optimized the suitable method of Lower Shihezi Reservoir, established a calculation method that the porosity of reservoir increased in the process of dissolution by applying the effect simulation, simulated porosity evolution of reservoir in research area and analyzed the porosity evolution model. The results showed that the reservoir of quartzose sandstone with moderate palaeogeothermal, long period of large buried-depth, carbonate cement was absent in early phase in the coal measure strata, and the porosity loss by compaction was substantial. Though secondary porosity was generated by dissolution, the quartz and clay mineral were precipitated from dissolution mass, which filled intergranular pores and dissolved pores. In addition to substantial porosity loss by compaction, the filling of massive quartz overgrowth, clay mineral, and ferrocarbonatite, always made such reservoirs to be low or ultra-low permeability reservoirs.