Brittleness evaluation is of great significance to the study of fracturing, excavation damage, and rock burst characteristics of rocks (especially deep rocks). The rock brittleness evaluation methods at home and abroad were summarized. Based on the limitation of the existing brittleness index in quantitatively characterizing the brittleness of rock, the statistical damage constitutive model which can simulate the whole process curve of rock stress-strain was considered, and the damage evolution characteristics of rock were analyzed. A new method for evaluating the brittleness characteristics of rock based on statistical damage constitutive relation was proposed. The brittleness index B_D was established to quantitatively characterize the brittleness of rock, and the rationality of the brittleness index was verified by model theory demonstration and laboratory triaxial compression test. Results show that the brittleness index B_D could well characterize the brittleness of rock, and the rock brittleness evaluation method based on statistical damage constitutive relation had good applicability. The correctness of the brittle index B_D and its superiority compared with other methods were verified by physical experiments of marble, granite, and Tamusu clay rocks under different confining pressures. Moreover, as confining pressure increased, Tamusu clay rocks showed obvious characteristics of transition from brittle failure to plastic failure.