To mitigate blasting vibration during the excavation of a drainage tunnel located 2.30~3.10 m beneath an existing tunnel, an optimized blasting scheme using millisecond blasting by electronic detonators and a subsection in blasting holes was implemented. The field blasting scheme was initially adjusted based on the conventional blasting situation near the existing tunnel. This involved optimizing hole position parameters and reducing the number of holes. Before the formal blasting in the underpass section, a single-hole blasting test was then conducted near the excavation face to capture the vibration waveform and geological information. Using the linear superposition method, the vibration waveform of various delay intervals was analyzed to select the optimal delay interval. To further improve blasting performance and reduce the vibration of the cut blasting, the first blasting in the cut area was performed by using the subsection blasting in the hole. Field tests and calculations determined that the optimal delay times were 5 ms for the same row of cut holes or spreader holes, 40 ms between rows, and 3 ms for contour holes. The new blasting scheme was implemented and optimized in the field. When the drainage tunnel was excavated at a footage of 1.5 m through the existing tunnel, the maximum vibration of the road surface monitoring point at a distance of 3.10 m directly above was maintained below 4.0 cm/s, ensuring structure safety. Using electronic detonators for precise initiation and sectional blasting successfully controlled site vibration, protected adjacent structures, and provided valuable insights for similar future projects.