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Understanding the effect of water activity on shale is of major importance for the exploration and development of shale gas reservoirs, especially in the drilling and completion processes. Deep brittle shale samples from the lower Silurian Longmaxi Formation located in Southwest China were characterized using X-ray diffraction (XRD), swelling ratio tests, computerized tomography (CT), Field Emission Scanning Electron Microscope (FE-SEM) and uniaxial compressive strength (UCS) tests. Because of the physical characteristics of the samples, the relationship between water activity and swelling ratio could be approximated by the Frenkel–Halsey–Hill (FHH) equation. The relationships between clay minerals, UCS and water activity were investigated. Based on the physical description of the internal structure, the impact of water activity on micro-fractures was also discussed. The results showed that multilayer adsorption was the main contributor to the swelling of the deep brittle shale samples. The swelling ratio of brittle shale samples was described by the water activity of external environment via a logarithmic expression of the FHH equation, with fractal dimensions ranging from 2.71 to 2.75. The mineralogical composition was a controlling factor on water activity, as shown by the positive correlation between water activity and the total content of clay minerals (TCCM), especially the amount of montmorillonite in illite/smectite mixed layer clay. When soaking in a solution with a water activity above the critical water activity, the brittle samples developed micro-fractures because the clay swelled, acting as cementing material in the weakened planes. The environmental water activity was used to evaluate mechanical properties of the hydrated brittle shale. Brittle shale samples with larger water activity had a higher critical water activity. Water activity analysis led to a better understanding of the characteristics of the brittle shale and the performance of the downhole fluid. [ABSTRACT FROM AUTHOR]