The true triaxial compression test was performed to further investigate the triaxial mechanical properties of coral concrete. Considering three concrete strength grades and nine stress ratios as variables, 100 mm × 100 mm × 100 mm cubic specimens were tested using a servo-controlled static-dynamic triaxial machine (TAWZ-5000/3000). The failure modes, stress–strain curves, peak stress, and peak deformation of coral concrete were obtained from the tests. The results showed that there are plate splitting and slant-shear in the failure modes of coral concrete. The stress and deformation were influenced by the concrete strength and stress ratio. The triaxial strength of coral concrete was significantly improved compared to the corresponding uniaxial strength. With the quadratic regression analysis, a functional relationship between material parameters and uniaxial compressive strength was established, which can accurately predict the meridian criterion of coral concrete for a certain strength grade. Moreover, comparing theoretical and experimental data, the failure criterion proposed by Wang for lightweight aggregate concrete was further verified to be applicable to coral concrete.