Abstract: The appropriate selection of inter−row delay time in buffer blasting directly affects the fragmentation distribution and boulder percentage of the muck pile. Based on the stress wave propagation theory and the prolonged action of detonation gas, this study conducted comparative field blasting tests in the context of deep−hole bench buffer blasting at an open−pit pyrite mine. With a fixed inter−hole delay time of 42 ms, four inter−row delay time schemes (100, 125, 150, 175 ms) were designed. Fragmentation distributions of the muck piles were acquired using UAV oblique photogrammetry combined with WipFrag image analysis, supplemented by blasting vibration monitoring, to systematically investigate the influence of inter−row delay time on fragmentation and boulder percentage. The results indicate that, with increasing inter−row delay time, the boulder percentage first decreases and then increases. At an inter−row delay time of 150 ms, the boulder percentage reaches its minimum value of 8.3%, indicating the optimal fragmentation effect. An excessively short delay time (100 ms) results in insufficient superposition of stress waves and uneven energy distribution, whereas an excessively long delay time (175 ms) weakens the inter−row synergistic fragmentation effect; both conditions are detrimental to rock fragmentation. Blasting vibration monitoring demonstrates that the vibration velocities for all schemes are substantially below the safety limits specified in the Safety Regulations for Blasting (GB 6722—2014), indicating that adjustments to the delay time do not produce significant vibration impacts on the surrounding environment. The optimal inter−row delay time of 150 ms identified in this study can serve as a reference for similar buffer blasting projects.