Abstract: Coral sand is the preferred backfill material for reef foundation engineering due to its complex particle morphology and highly porous structure. The mechanical properties of coral sand deposits exhibit significant anisotropy resulting from gravitational sedimentation processes and inherent particle characteristics. This investigation conducted true triaxial drained shear tests on South China Sea coral sands to systematically evaluate the influence of intermediate principal stress coefficient and initial deposition angle on mechanical responses. Experimental findings reveal two critical trends: (1) Peak shear strength demonstrates marked enhancement with increasing intermediate principal stress coefficient, accompanied by reduced dilatancy effects; (2) Progressive attenuation of peak strength occurs with elevation of initial deposition angle. A novel three-dimensional anisotropic constitutive model was developed within the theoretical framework of bounding surface plasticity. Model validation through comparative analysis with true triaxial test data confirms its predictive capability. These research outcomes establish a theoretical basis for evaluating three-dimensional bearing capacity characteristics of coral sand foundations in South China Sea reef engineering applications.