Abstract: Aiming at the challenges of deep-marine shale gas development in the southern China, and taking the Wufeng-Longmaxi shale in the Sichuan basin as the research object, the paper focuses on the brittleness analysis and characterization of deep marine shale, establishes a quantitative assessment index of shale brittleness based on the complete stress-strain curve, and uses the peak strain and steepness of post-peak slope as indicators to express the degrees of brittle failure complexity and adequacy, respectively. Then according to the results of high-confining-pressure triaxial compression tests for main shale lithofacies including siliceous shale, calcareous-siliceous shale, calcareous shale and clay-siliceous shale, the paper analyzes the influence of confining pressure on shale brittleness, establishes a brittleness evaluation method for deep marine shales and compares the brittleness of deep and shallow shale reservoirs. The results show that under low confining pressure, the pressure mainly enhances the compressive strength of the shale and the shale brittleness increases with the enhancement of the confining pressure; while under high confining pressure, the pressure mainly restrains the initiation and propagation of fractures and the shale brittleness decreases with the weakening of the confining pressure; using elastic parameters as the indicators of lithology and fracture toughness as an indicator of confining pressure, a mechanical brittleness index is established, which can be effectively used in the brittleness evaluation of the deep shale reservoirs below 3 500 m as the supplementary of the mineral method and elastic parameter method; the siliceous shale and calcareous-siliceous shale still have the potential to be favorable reservoirs at the depth of 4 500-5 000 m. However, the deep marine shales have thinner “sweet spot” layers and lower development potential due to the brittleness weakening