Vertical deep hole underground blasting operations exhibit significant sensitivity to stemming length due to constrained free surface and singular energy release pathways, with this parameter critically influencing both explosive energy efficiency and fragmentation characteristics. This study investigates the impact of stemming length on vertical deep-hole blasting through theoretical analysis, scaled experiments, LS-DYNA simulations, and field validation, with theoretical results indicating that optimal stress-wave preservation occurs at a 5 cm stemming length when using a 5 cm burden distance. The experimental study employed concrete specimens with a constant burden distance of 5 cm and tested four distinct stemming configurations: 0 cm, 2 cm, 4 cm, and 5 cm. The results show that both the blasting crater radius and ejecta mass initially increased, then plateaued as the stemming length increased, achieving optimal performance at 5cm with a crater diameter of 7.9 cm, an ejecta mass of 207.96 g, and the most uniform fragmentation distribution. Field trials employing a 1.0 m stemming length after similarity ratio conversion demonstrated uniform fragmentation without cratering effects, thereby validating the 5 cm optimal length determination. These findings establish both theoretical foundations and practical guidelines for enhancing vertical deep-hole blasting performance in underground operations.