This paper presents a new method for recovering paleoporosity of sandstone reservoirs and quantitatively defines the evolution process of porosity. This method is based on the principle that the present is the key to the past. We take the middle Es3 member in Niuzhuang Sag, Dongying Depression, and Bohai Bay Basin as an example. The method used in this study considers the present porosity as a constraint condition, and the influences of both constructive diagenesis and destructive diagenesis to divide the porosity evolution process into two independent processes, namely porosity increase and porosity decrease. An evolution model of sandstone porosity can be established by combining both the pore increase and pore decrease effects. Our study reveals that the porosity decrease model is a continuous function of burial depth and burial time, whereas the porosity increase model mainly occurs in an acidified window for paleotemperature of 70°C to 90°C. The porosity evolution process can be divided into the following phases: normal compaction, acidification and pore increase, and post-acidification compaction. Thus, the porosity evolution model becomes a piecewise function of three subsections. Examples show that the method can be applied effectively in recovering the paleoporosity of sandstone reservoirs and simulating the porosity evolution process.