Construction method for composite stratum with hard top and soft bottom of large-section tunnel

The invention discloses a construction method of a railway tunnel, and particularly relates to a construction method for a composite stratum with the hard top and the soft bottom of a large-section tunnel. The method includes excavation construction; initial-stage supporting; and inverted arch and lining construction. When the thickness of a hard surrounding rock on the upper portion of a working surface of the tunnel is smaller than 2m in an excavation construction process, the excavation construction method is a three-step seven-pace method; and when the thickness of the hard surrounding rock on the upper portion of the working surface of the tunnel is larger than 2m, the excavation construction method is a three-step method. The construction method has the advantages of fast construction progress, high safety and fine construction economical efficiency.