Carbonate reservoir in Dengying formation of Sichuan Basin has huge resource reserve. This carbonate is mainly characterized by low porosity and low permeability, thus hydraulic fracturing is the necessary technology for its development. Whereas, the complexity of carbonate mechanical property makes hydraulic fracturing have low efficiency, impeding the gas production. In particular, as an important mechanical parameter, brittleness is the key factor of fracturing design and optimization. Currently, no appropriate brittleness evaluation method has been built for carbonate reservoir in Dengying formation. Therefore, this paper investigates the mechanical characteristics of carbonate by conducting triaxial compression, Brazilian splitting, and fracture toughness tests. Based on these basic mechanical parameters, the brittleness and plasticity of carbonate are fully analyzed. Consequently, a novel brittleness has been established including compressive-tensile-fracture characteristics. Results indicate both tensile strength and fracture toughness show certain heterogeneity. In deep formation environment, with increasing temperature, confining stress and pore pressure, the whole strength increases but deformation has been restricted. Specially, with strong restriction effect on rock failure from confining stress, carbonate brittleness has clear decline and also shows increasing plasticity. Furthermore, under the influence of acid fluid, rock strength declines and deformation have small increment. Since structure damage occurs under the influence of acidification, rock has larger failure degree and show stronger brittleness. Outcome of this paper deepens the understanding of carbonate brittleness and offers theoretical reference for hydraulic fracturing in carbonate reservoir.