In tunnel blasting operations, the performance of auxiliary holes following initial cut blasting critically influences subsequent excavation progress and operational efficiency. The residual borehole length after auxiliary-hole detonation is a crucial parameter for assessing both the efficiency of blasting energy utilization and the effectiveness of rock fragmentation. This study investigates the impact of the residual borehole length after auxiliary-hole blasting in hard-rock tunnels on subsequent excavation performance, using the Fangshan Tunnel as a case study. A three-dimensional dual-borehole model was developed in Hypermesh with localized mesh refinement around blast holes. LS-DYNA simulations were conducted to analyze residual hole lengths under varying inter-hole delay times and geological conditions, with key parameters systematically configured for blast dynamics analysis. This research systematically examines the governing patterns of inter-hole delay time and surrounding rock mass on the residual borehole length in auxiliary holes, employing advanced numerical simulation techniques for comprehensive parametric analysis. The results demonstrate that the inter-hole delay time significantly influences residual borehole length, with longer delay times within a specific range increasing residual length and accelerating growth rate. Furthermore, the lithology of the surrounding rock mass emerges as a critical determinant of residual length, with granite exhibiting longer residual holes than sandstone under identical charging conditions, revealing a positive correlation between the rock firmness coefficient and residual hole dimensions. Field verification was conducted using optimized auxiliary hole blasting parameters derived from this study. The implementation of these parameters in actual construction effectively controlled the residual borehole length and significantly enhanced blasting performance in hard-rock tunnel excavation. The findings establish both theoretical foundations and methodologies for precision design and dynamic adjustment of auxiliary hole blasting parameters in hard rock tunnel excavation, offering substantial value for enhancing construction efficiency and optimizing project economics in tunneling operations.