Investigation of the porosity variation of oil shale during the pyrolysis process is of great importance in understanding the migration mechanism of pyrolysis products. Using the Fushun oil shale of China as an example and taking into account the fact that the porosity change of oil shale arises from pyrolysis of kerogen at high temperature, in this paper, the thermal curve of oil shale was employed to obtain its pyrolysis rate equation. A quantitative model of oil shale porosity during pyrolysis was constructed. The porosity of oil shale under different pyrolysis conditions was determined using the conventional method, computed tomography (CT) and the mercury intrusion method. The calculated results were in good agreement with the experimental data.