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Abstract The mechanical properties of deep volcanic reservoir rocks are complex and the brittleness characteristics are not clear. In order to study the brittleness characteristics of deep volcanic rocks, triaxial compression tests and XRD tests under different confining pressures were carried out to study the brittleness characteristics of deep volcanic rocks. Three methods were used to evaluate the brittleness of volcanic rocks from different sides: the brittleness evaluation method based on stress–strain curve characteristics (BI1), the brittleness evaluation method based on energy dissipation (BI2) and the brittleness evaluation method based on mineral composition (BI3). The three methods showed that the brittleness index of volcanic rocks decreases with the increase in confining pressure, and the higher the depth of volcanic rocks, the lower the brittleness. On the basis of previous studies, an improved brittleness evaluation method based on internal friction Angle was proposed, and the obtained evaluation results were almost consistent with the results obtained by the brittleness evaluation method based on stress–strain curve characteristics (BI1) and the brittleness evaluation method based on energy dissipation (BI2), indicating the feasibility of the improved brittleness evaluation method. The brittleness characteristics of deep volcanic rocks are clearly defined to provide reference for the exploration and development of deep volcanic rocks reservoir.