The research on the evolution of sandstone's porosity is the indispensable basis for the analysis of buried history, the palaeotectonic reconstruction and the prediction of palaeo-formation pressure, and accurate acquirement of the primary porosity is the basic prerequisite to achieve the aim. The common method is the empirical relationship created by Beard using sorting coefficient, or the reading of the R.Sneider chart to get porosity and permeability relying on the coring experiment. Based on an analysis of the stress state of sediment particles, the authors set up the primary porosity calculation model with the average particle size ranges as the parameters. Using this model, the authors processed the reservoir sandstone of Chang 8 formation of the Triassic Yanchang Formation in Jiyuan area of Ordos Basin and Bashijiqike Formation in Dabei area of Tarim Basin. A comparison with the previous research results in corresponding areas has verified the reliability of the models.