Abstract: To address the problems of high ground stress, stringent contour precision requirements, and the proneness of traditional smooth blasting to over-break, under-break, and surrounding rock damage in deep-buried hard rock water diversion tunnels, a Class II metamorphic sandstone hard rock water diversion tunnel was taken as the engineering background in this study. A numerical model of double peripheral borehole blasting was established to analyze the influence of air-decking on inter-hole fracture formation and radial surrounding rock damage. The results showed that 450 mm was determined as the optimal air-decking, at which the best balance between fracture formation and surrounding rock protection was achieved, and the rock mass damage range was approximately 17 cm. Based on the above parameters, a “layered control, dynamic adaptation” smooth blasting technology system was constructed, which incorporated differentiated charge structures, a digital electronic detonator millisecond-delay initiation network, and full-process quality control measures, and a 200 m field test was carried out for validation. The results show that: in terms of contour quality, the average linear over-break is reduced by 21.6%, the local under-break is reduced by 44.8%, and both the borehole trace rate and utilization rate are increased by 7%; in terms of surrounding rock stability and construction efficiency, the explosive unit consumption is reduced by 16.7%, and the monthly excavation footage is increased by 25%, which achieves synergistic improvement in contour quality, surrounding rock stability, and construction efficiency. The research results can provide technical reference for the design and construction of smooth blasting in deep-buried hard rock water diversion tunnels.