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Clarifying the evaluation methods and factors influencing shale brittleness is of great practical significance for fracture mechanism analysis, rock mass stability evaluation, geophysical interpretation, ”sweet spot” engineering optimization, and exploration risk assessment. This paper comprehensively analyses and discusses the evaluation methods and factors influencing shale brittleness. This study shows that the methods for evaluating shale brittleness mainly include the rock stress–strain relationship, brittle minerals, and comprehensive evaluation methods. The factors influencing shale brittleness include internal geological and external environmental factors. The internal factors are mainly the type and content of brittle minerals, the content and maturity of organic matter, rock bedding, porosity, etc. In contrast, the external factors are the temperature and confining pressure environment under the controls of burial depth and horizontal [geostress] differences. These factors control the stress–strain characteristics, mechanical properties, and fracture characteristics of shale to different degrees, thereby affecting shale brittleness. Therefore, the combination of experimental testing and geophysical interpretation, the development of dynamic visualization technology, and the establishment of a geological evaluation model should be given more attention in the future to provide a reference for shale compressibility evaluation and optimization of favourable fracturing target intervals.