<p>The intermediate principal stress and unloading action have a significant impact on the mechanical properties and strength criterion of rock. The physical experiment of one-sided lateral unloading—three-direction and five-face stress—vertical continuous loading with different intermediate principal stresses is conducted. The change rule of intermediate principal stress on the deformation and strength characteristics of tested specimen is clarified. The effect of intermediate principal stress coefficient on the friction angle and cohesion of tested specimen is explained. The evolution characteristics of stress Lode angle and strain Lode angle under different conditions are revealed. The impact of intermediate principal stress and unloading action on the strength criterion of tested specimen is analyzed. The findings indicate that the stress–strain curve of tested specimen may be categorized into the initial compaction stage, elastic deformation stage, crack stable fracture stage and crack unstable fracture stage. The initiation and failure stress of tested specimen show an approximately linear increase by the increase of intermediate principal stress. The cohesion decreases and the angle of internal friction increases by the increase of intermediate principal stress coefficient. At the peak, both the Lode angles of stress and strain are higher for the unloading condition than for the loading condition. And the difference of Lode angle in both conditions increases by increasing the intermediate principal stress. The intermediate principal stress and unloading effect may reflect the rock yielding properties by the stress Lode angle. The strain Lode angle is used to reflect the evolution direction of fracture in tested specimen. The Mohr–Coulomb criterion has obvious linear characteristics. The deviatoric stress of Drucker–Prager criterion is not affected by the stress Lode angle. The SMP criterion is more adequate to strength characterization of deep rock with true three-dimensional stress.</p>