To minimize the blasting influence of the rear tunnel of multiarch tunnel without middle pilot heading on the secondary lining structure of the first tunnel, a case study is conducted on the Xinmin tunnel in Yunnan, China, and the influence of the rear tunnel blasting on the secondary lining structure of the first tunnel under weak and broken surrounding rock is investigated using field monitoring and numerical simulation methods. To prevent the secondary lining structure of the first tunnel from large cracks induced by blasting, the millisecond delay blasting technology between cut holes is adopted to optimize the original blasting scheme on the basis of reducing the single stage blasting charge and optimizing the blasting sequence. The measured data shows that under the original blasting scheme, the radial blasting vibration velocity exceeds 32 cm/s; after blasting scheme optimization, the radial blasting vibration velocity caused by the cut hole is less than 8 cm/s, indicating a good optimization effect. The finite element simulation results reveal the following: (1) The maximum absolute vibration velocity of the first tunnel is mainly radial during the blasting process, the arch shoulder is most affected by the blasting, and the concrete structure will be subject to the tensile force in the direction of the tunnel interior. (2) The millisecond delay blasting technology between cut holes can slightly reduce the impact of blasting on the lining structure of the first tunnel without reducing the maximum charge of blasting and changing the blasting sequence. According to the monitoring and field observation, the blasting optimization measure recommended is reasonable and feasible.