In order to predict the hydrogeological environment changes brought by in-situ pyrolysis process, Controlling the pyrolysis temperature and time was used to investigate the porosity change law of oil shale with different degrees of pyrolysis. Starting from the mechanism of oil shale porosity change caused by kerogen pyrolysis, combined with the pyrolysis reaction rate equation of oil shale, a quantitative model of porosity variation of oil shale during pyrolysis was established, and the theoretical and experimental values were compared and verified. The results show that the porosity data agrees well and the error is small. It is concluded that the quantitative model of porosity can accurately calculate the porosity of oil shale during pyrolysis.