The evolution of pore structure during in situ underground exploitation of oil shale directly affects the diffusion and permeability of pyrolysis products. In this study, on the basis of mineral analysis and thermogravimetric results, in combination with the low-pressure nitrogen adsorption (LPNA) and mercury intrusion porosimetry (MIP) technique, the evolution of pore structure from 23 to 650 °C is quantitatively analyzed by simulating in situ pyrolysis under pressure and temperature conditions. Furthermore, based on the experimental results, we analyze the mechanism of pore structure evolution. The results show the following: (1) The organic matter of Fushun oil shale has a degradation stage in the temperature range of 350⁻540 °C, and there is no obvious temperature gradient between decomposition of kerogen and the secondary decomposition of bitumen. The thermal response mechanisms of organic matter and minerals are different in each temperature stage, and influence the change of pore structure. (2) Significant changes occur in pore shape at 350 °C, where thermal decomposition of kerogen begins. The ink-bottle pores are dominant when the temperature is less than 350 °C, whereas slit pores dominate when the temperature is greater than 350 °C. (3) The change in pore structure of oil shale is much less significant from 23 to 350 °C. The pore volume, porosity, and specific surface area (SSA) of samples increase rapidly with temperature varying from 350 to 600 °C. The variation of each parameter is dissimilated from 600 to 650 °C: the porosity and pore volume increases with a small gradient from 600 to 650 °C, and SSA decreases significantly. (4) The lithostatic pressure does not cause change in the evolution discipline of the pore structure, but the inhibitory effect on the pore development is significant.