A series of research on mechanical behaviors of big pipe-roof in shallow large-span loess tunnel was made on Wenxiang Tunnel in Zhengzhou-Xi'an Special Passenger Railway. The longitudinal deformations of the pipe-roofs are monitored and the mechanical behaviors of the pipe-roofs are researched at the test section. The force-deformation law of big pipe roof in loess tunnel was obtained: in front of the excavating face about 15 m, the pipe roof started to bear the load; after the excavating face about 15 m,the load became stable; the longitudinal deformation of the whole pipe-roofs was groove-shaped distribution; the largest force of pipe-roofs was at the excavating face. The conclusions can provide reference for the design parameter optimization and the construction scheme selection of pipe-roofs and had been verified in the result of numerical analysis software FLAC3D and deformation monitoring.