Currently, the mechanical properties of calcareous sand are mainly studied through triaxial tests, as traditional uniaxial compression tests fail to capture real loading conditions and soil strength anisotropy. To address this, true triaxial tests were conducted to examine the effect of the intermediate principal stress parameter (b) on the three-dimensional strength and deformation behavior of calcareous sand. In the constant b and sigma 3 tests, as the b value increased, both the strength and peak friction angle (phi ps) of calcareous sand were increased, while the tangent slope of the dilatancy curve showed a gradual rise.. The phi ps of calcareous sand was found to be higher compared to silica sand and coarse-grained soils. In the constant mean effective stress (p) and b test, the strength was increased with higher values of both b and p. The Matsuoka-Nakai 3D strength criterion proved more effective in fitting the 3D strength of calcareous sand in pi plane. As the b value increased, the critical stress ratio (Mc) was decreased. A quadratic function can better represent the Mc of calcareous sand in the pi plane under varying confining pressures. Furthermore, the Mc of calcareous sand was higher than that of silica sand and completely decomposed granite soil. This study provides a valuable experimental basis for understanding the 3D strength and deformation characteristics of calcareous sand in oceanic engineering infrastructure.