Relying on the construction under crossing the existed south Tongpu Railway with a small included angle at Gaoqiao tunnel of Zhengzhou-Xi’an high speed rail, carrying out the test and study, the mechanical characteristics and key construction techniques of the double-layer support system of large-section loess tunnels by benching method are acquired. It successfully resolved the construction difficulties of super-large-section tunnel (171m₂) under crossing the operating railway in the depth of only 11m under sandy neoloess, thus to have the significant reference for the design and construction of the similar engineering.