Preceding the in-situ pyrolysis of oil shale, it is imperative to undertake reservoir fracturing and modification. The intricate evolution of meso-structures in both the fractured fissures and the surrounding rock matrix, constrained by in-situ stress during the pyrolysis process, plays a significant role in influencing the efficiency of heat transfer and the extraction effectiveness of produced materials.This article is based on the Micro-CT scanning technology to study the microscopic structural evolution patterns of high-temperature water vapor in-situ pyrolysis in oil shale with fractured fissures.The main research findings are as follows: Firstly, pre-existing fracturing fissures facilitate the formation and development of vertical bedding fractures.The vertical bedding fractures first appear at 300 °C, with a relative frequency of 1.35%, occurring earlier than the temperature at which such fractures emerge in intact oil shale.Secondly, at 450 °C,there is a pivotal transition point in the evolution of pore-fracture systems, shifting from a predominant increase in length to a predominant increase in aperture.Below 450 °C, changes in pore-fractures are primarily manifested in terms of quantity and length. However, after 450 °C, there is a noticeable increase in the aperture of pore-fracture systems.Thirdly,at temperatures within the range of 300 °C–350 °C, a closure phenomenon occurs in fractures, predominantly involving micro and medium fractures. This closure predominantly affects smaller fractures, providing only a moderate retardation effect on the development of larger fractures. Moreover, in the temperature range of 500 °C–550 °C, there is interconnection observed between large and medium fractures. Pre-existing multiple large fractures are consolidated into a singular interconnected large fracture.Finally, at 450 °C, there is a significant increase in the connectivity of pore-fracture networks. Below 450 °C, the primary connected flow pathways consist of pre-existing fracturing fissures and newly formed parallel bedding fractures. Beyond 450 °C, a complex network of pores takes over as the main interconnected flow channel.