Based on analysis of tight sands reservoir characteristics and cotrolling factors, two types of porosity decrease mathematical model and one type of porosity increase mathematical model in the study area were established by phases. With geological time and burial depth as variables, current porosity as boundary constraint conditions, burial stages and key diagenesis window analysis as basis, the whole porosity evolution process from initial burial until nowadays was divided into seven separate functions which repeated porosity evolution history of the study area dynamically. The quantitative simulation result indicates that: the porosity evolution of each stage follows porosity decrease model or superimposion model of two different types of porosity-evolution model; the variable of porosity decrease model of mechanical compaction stage is burial depth, while variables of compaction-cementation comprehensive stages are geological time and burial depth; porosity increase model is applicable within dissolution window which is identified by Ro from 0.6 to 1.0; the main reason reducing the porosity greatly is mechanical compaction, followed by compaction-cementation, dissolution improves pore space to some extent, and later compaction-cementation causes porosity decrease further until tight reservoir nowadays.