Abstract: The currently widely used asphalt mixture failure criteria fail to adequately reflect the influence of intermediate principal stress on the material strength, leading to limitations in traditional asphalt pavement structure design. Based on true triaxial loading equipment, this study designed high- and low-temperature true triaxial tests for asphalt mixtures. The tests employed an isostress load path with a load angle θ, measuring triaxial stress data at shearing stress parameters μ of 0.00, 0.25, 0.50, 0.75, and 1.00. Based on the unified strength theory and the double shearing element model, the study analyzed the intermediate principal stress effect on the shearing strength of asphalt mixtures and established a three-dimensional strength criterion. The research results indicate that: Under true triaxial stress conditions, the stress-strain curve of asphalt mixture exhibits significant non-decaying hardening characteristics, and the stress development (σ₁) continues to increase with strain (ε₁), entering a plastic flow state under high pressure; The double-shear unit cell model effectively characterizes the triaxial stress characteristics of asphalt mixtures under high and low temperatures, as the double-shear stress state parameter μ increases from 0~1, the peak octahedral shear stress τ_oct increases by approximately 2 MPa~4 MPa, and the double-shear stress τ_tw exhibits a linear relationship with the corresponding normal stress σ_tw; The π-plane limit trace under different hydrostatic pressures ξ lies between the unified strength theory traces for b=0 and for b=1, and based on comprehensive multi-condition data, the unified strength yield surface with b=1/2 can optimally fit the triaxial stress state under high and low temperatures and under different hydrostatic pressure conditions. The research findings can provide a theoretical support for asphalt pavement structure design and for a rutting prediction.