Abstract: To effectively mitigate bedrock damage caused by blasting excavation in sea-crossing projects, this study systematically investigates the damage control effectiveness of a composite bottom cushion charging structure in underwater drilling blasts. By integrating a case study of embedded open caisson blasting excavation for a sea-crossing bridge, field sampling and underwater explosion model tests were conducted. The influence of iron-sand concrete with different wave impedances in the composite bottom cushion on rock sample damage was quantitatively analyzed using a piezoelectric ceramic detection system. Based on fractal dimension and damage theory, the propagation behavior of cracks on the top of rock samples under various working conditions was quantitatively evaluated. Results indicate that the composite bottom cushion effectively suppresses the propagation of macroscopic cracks in rock samples, reduces the number of top cracks, and decreases the axial propagation depth of cracks. Analysis of axial damage in rock samples shows that with increasing wave impedance, the maximum axial damage factor in the borehole region (0-12 cm) can be reduced by up to 9.95%, while the reduction in the bedrock region (12-28 cm) reaches 40.23%-92.1%. The research demonstrates that employing a composite bottom cushion charging structure in underwater drilling blasting can significantly alleviate blast-induced damage to bedrock, and effectively control axial damage in rock samples by adjusting the wave impedance of iron-sand concrete.