Abstract: To address the problems of high boulder yield and severe harmful vibration effects frequently encountered in practical decked blasting, this paper aims to investigate the influence of in-hole millisecond delay variation on decked blasting and thereby improve its performance. Through model experiments combined with vibration monitoring and fragmentation analysis, the effects of in-hole delay variation on blast vibration, fragmentation, and other blasting outcomes were explored. ANSYS numerical simulation was employed to reveal the damage evolution and rock mass throwing process induced by decked blasting with in-hole delays. Guided by the patterns obtained from the model experiments and numerical simulations, a limited number of field verification tests were conducted to confirm the feasibility and advantages of in-hole millisecond delays under actual working conditions. The experimental results indicate that under identical open-pit mine conditions, compared with decked blasting without delay, the scheme in which the lower deck initiates first and the upper deck is delayed by 8 ms reduces the boulder yield by 3.05%, decreases the near-field blast vibration by 37.98%, and minimizes toe residuals. The scheme in which the upper deck initiates first and the lower deck is delayed by 6 ms reduces the boulder yield by 4.19%, decreases the near-field blast vibration by 40.02%, and achieves better fragmentation and throwing effects. It is confirmed that setting reasonable in-hole millisecond delays for decked charges based on actual conditions in open-pit deep-hole blasting can effectively improve energy utilization efficiency while reducing boulder yield and the harmful effects of blast vibration.