Abstract: Convective heating technology is one of the key feasible technologies in the in-situ retorting of oil shale. The use of steam as a heat-carrying fluid has been shown to have significant advantages in terms of technical process and economy. In deep high-stress environments, low-pressure high-temperature steam can be transformed into near/supercritical water. In this study, experiments were performed on oil shale pyrolysis through anhydrous conduction heating and near/supercritical water heating. Combined with micro-CT scanning, triaxial permeability tests, and uniaxial compression mechanical tests, an in-depth comparative study was conducted on the evolution of microscopic pore and fracture structures of oil shale as well as the macroscopic permeability and mechanical changes under different pyrolysis environments, thus explaining the differential mechanisms of macro and micro oil shale characteristics induced by different pyrolysis conditions. The porosity and fracture development degree of oil shale under near/supercritical water pyrolysis conditions are much higher than those under anhydrous pyrolysis condition, with the porosity differences of nearly ten times. The maximum crack width is increased from around 30 μm to about 150 μm under near/supercritical water conditions. The changes in minerals after near/supercritical water pyrolysis significantly affect the microscopic structure of oil shale, forming numerous micro-nano pores between the illite-smectite mixed layers. Under different stress and pore pressure conditions, the permeability achieved by anhydrous pyrolysis is below 0.04 mD, while the permeability under near/supercritical water conditions reaches 0.1-0.7 mD, differing by nearly 20 to 50 times. By contrast, supercritical water has stronger permeability, heat transfer capacity, solubility, and carrying capacity. In the supercritical water pyrolysis environment, oil shale forms numerous fractures along parallel beddings and interlaminar cross fractures connecting beddings, shown as more complex microscopic structures and smoother flow channels. The compressive strength of oil shale under near/supercritical water conditions is significantly lower than that in an anhydrous environment at the same temperature. The participation of near/supercritical water accelerates the decomposition of organic matter, causing oil shale to delaminate at a lower temperature. The results provide a theoretical basis for the convective heating exploitation of deep oil shale and other organic-rich rocks.