Based on the Longquanshan Tunnel in Chengdu, numerical simulation calculations are conducted using the equivalent load method to investigate the dynamic response characteristics of the existing tunnel structure under different excavation sequences and clearances. The main research conclusions are as follows: (1) Due to the lack of a free surface, the excavation sequence of the subsequent tunnel has the greatest impact on the lining structure, while the impact of blasting sequences 2 and 3 on the structure is relatively small. (2) Under different excavation sequences, the peak vibration velocity of the subsequent tunnel structure is mainly observed at the sidewall, arch crown, and springing positions on the blasting face, with a maximum monitored velocity of 5.665 cm·s^-1. (3) The comprehensive peak vibration velocity in the reverse excavation direction of the preceding tunnel generally decreases, while along the excavation direction, the peak vibration velocity shows a tendency to increase within a certain range. (4) Under different sequences, the peak vibration velocity of the subsequent tunnel structure occurs at the left arch crown, left side voussoirs, and right shoulder of the arch, with a maximum monitored velocity of 5.387 cm·s^-1. (5) The vibration velocity on the blasting face is sensitive to the clearance, while the peak vibration velocity on the opposite blasting face shows less significant changes with a variation within 2.5%. (6) Under the same clearance condition, the maximum vibration velocity in the X direction at the monitoring point on the blasting face of the preceding tunnel is the highest, followed by the Y direction, and the Z direction is the lowest. (7) The natural frequency of the structure near the right sidewall of the tunnel may be between 14.25 and 18.0 Hz, and the structure is sensitive to stress wave responses in this frequency range.