Multi-arch tunnel in-situ extension construction method using existing lining

The invention relates to a multi-arch tunnel in-situ extension construction method using an existing lining. The method comprises the following steps: designing an excavation contour surface of an extension tunnel; backfilling the lower half part of the tunnel; partial lining of the vault on the outer side of the existing multi-arch tunnel is removed, a vault rock-soil body is excavated, and a primary support is constructed; removing an outer side tunnel backfill soil body, dismantling an outer side tunnel side wall, excavating a rock-soil body behind the side wall, and constructing a primary support; partial linings of the arch top of the tunnel on the inner side of the existing multi-arch are dismantled, arch top surrounding rock is excavated, and inner side primary supports are constructed; removing backfill soil of the inner side tunnel; removing residual lining structures and steel supports, excavating to a tunnel design contour line position, constructing an inverted arch, and taking the existing centra