Abstract: This study focuses on the development of unconventional energy resources in deep subsurface environments, where a specialized ultra-low permeability testing system has been independently developed. The system is tailored to reservoir characteristics and extreme burial conditions, aiming to solve the challenges of accurately determining ultra-low permeability in dense rocks under high geothermal temperatures, high geo-stress, and high pore pressures. The system comprises two key components: a flow pipeline and a high-temperature, high-pressure multi-channel pressure chamber. Through innovative design and system optimization, it reliably simulates deep reservoir environments with temperatures up to 150 ℃, confining pressures of 150 MPa, and pore pressures of 100 MPa. Additionally, it measures the permeability of rock samples ranging from sub-nanodarcy to millidarcy levels using the pulse decay method. Based on this system, we have conducted permeability tests on granite and shale under various conditions, including different loading and unloading cycles, effective stress, pore pressure, and temperature. The results indicate that increased loading and unloading cycles help reduce the impact of secondary deformation or micro-fractures during sampling and preparation on the test results; increased pore pressure suppresses the gas slippage effect, leading to apparent permeability measurements that more closely reflect the intrinsic permeability. Under identical effective stress conditions, the permeability of a single fracture in shale is at least three orders of magnitude higher than that of the matrix. The developed physical model for fracture permeability accurately fits the experimental results and describes the permeability variations in fractured shale under the combined effects of effective stress, pore pressure, and fracture compression deformation. Furthermore, under fixed conditions of effective stress and pore pressure, shale permeability decreases as temperature increases. In summary, the developed ultra-low permeability testing system provides technical support for permeability evaluation in tight reservoirs, and the relevant test results contribute to a deeper understanding of fluid migration mechanisms in deep reservoir environments.