In constructing shield tunnels in a sea area, large-sized boulders and bedrock are often encountered, necessitating pretreatment via blasting. The effectiveness of blasting pretreatment is crucial for the regular excavation of shield machines. Based on Xiamen Metro Line 2 project, a refined blasting pretreatment method for boulders and bedrock in shield tunnels under the sea area is proposed. The method comprehensively considers overburden conditions, blasting fragmentation indexes, and marine biological safety standards. Specific steps include designing blasting parameters, calculating powder factor, determining single-hole charges and average block size, designing charge structures and initiation networks, predicting the distribution of blasting fragments, and optimizing the blasting program to minimize ecological impact. Field application results indicate that post-blasting fragment sizes are within 30 cm, meeting the size requirements for the shield machine. The shield machine could excavate smoothly through the blasting pretreatment section, with excavation parameters similar to those in regular sections. The proposed method achieved a refined, ecological, efficient and safe blasting construction in the sea section containing boulders and bedrock.