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As a basic mechanical property of rocks, brittleness is closely related to the drillability, wellbore stability, and rockburst characteristics of reservoir rocks. Accurate evaluation of rock brittleness is of great significance for guiding oil and gas production and reservoir reconstruction. This paper systematically introduced the commonly used brittleness evaluation methods based on the stress–strain curve and analyzed their theoretical background and mathematical models. Combined with practical engineering application, the characterization effect of commonly used brittleness indexes in various rock samples is verified and optimized, and it is obtained that the brittleness index (B17 in this paper), based on the stress–strain curve and considering energy conversion, has the best characterization result for rock brittleness, which has good differentiation for different rock samples. At the same time, considering that the stress–strain curve under high confining pressure may result in a significant yield plateau phenomenon before and after the peak strength, the endpoint of the plastic yield plateau is used to replace the peak point as the starting point for the drop of bearing capacity. The revised brittleness index is consistent with the changing trend of the original curve, which verifies the reliability of the model. Finally, the method for characterizing the brittleness of Class II curves is supplemented, and the combined brittleness index of rock is established, which verifies the rationality and correctness of the index and provide a more general evaluation method for rock brittleness in engineering.