Abstract:In this paper, a new construction monitoring method for large cross-section tunnels in underground projects and a new astringe surveying rod developed are introduced. And the application of monitored data in the wall rock back analysis, convergence law and loose loops for large Cross-section Tunnels is discussed. On the basis of some actual projects, such parameters as wall-rock loose loops and unsupported roof span are provided for Qianling Iron Mine, which can be used as reference and guide for similar projects.