The causes of surrounding rock rheology and lining crack of ridge core section of Muzhailing Tunnel on LanzhouChongqing Railway are analyzed so as to solve the problem of serious deformation under high geostress. And then, the dismantling and enlarging technologies for Muzhailing Tunnel ridge core section are proposed, i.e. adding cover arch reinforcement, radial grouting of surrounding rock, strengthening the support rigidity, optimizing the tunnel crosssection, controlling the construction step, and adjusting reserved deformation. The muck backfilling with mechanical excavation method is selected according to comparison between mechanical excavation method and bench method. The construction technology and construction organization of muck backfilling with mechanical excavation method are introduced; and the deformation and structural force of tunnel support are monitored. The monitoring results show that the deformation of support is in the permissible range; the structure of secondary lining is stable and the muck backfilling with mechanical excavation method is safe and feasible.