The particle breakage, deformation, and strength properties of calcareous sand are related to the drainage conditions during shearing. However, the effect of undrained shear processes on the particle breakage and mechanical properties of calcareous sand are rarely considered in the current studies. A series of consolidated drained and undrained triaxial shear tests is conducted on calcareous sand with various initial relative densities under different effective confining pressures to investigate the effects of drainage conditions on particle breakage and mechanical properties during shearing. The results show that the particle breakage rate in the drained shear test is higher than that in the undrained shear test under the same effective confining pressure. The stress–strain curves of calcareous sand exhibit the behavior of strain softening during both drained and undrained shear, and its dilatancy behavior is influenced by the initial relative density and the effective confining pressure. The critical state of calcareous sand is independent of its initial state but related to the drainage conditions during shearing. Both the critical stress ratio and the phase change stress ratio in the drained shear test are larger than those in the undrained shear test. The peak effective friction angle of calcareous sand decreases with the increase of initial relative density, effective confining pressure, and percentage of particle breakage while the critical effective friction angle is not affected by these three factors. Both of these effective friction angles are related to the drainage conditions during shearing. The peak effective friction angle and the critical effective friction angle in the drainage shear test are greater than those in the undrained shear test. The above results indicate that the particle breakage, deformation, and strength properties of calcareous sand are significantly correlated with the drainage conditions during shearing.