Research conclusions: Compared with the absolute measured deformation，all calculated deformations were in the controlled ranges stipulated in the codes and standards based on the pre—measured and simulated results．Therefore，for construction of ultra—high sectional and ultra—shallow—buried tunnel under the condition of weak—water rock．the establishment of ANSYS simulation model with 100 percent of the stress release by the plane strain mode could meet the demands of the process control and optimization．For construction of the tunnel with CRD method，the simulation results showed that settlement of 2，4，5 and 7 subsections accounted for 85～90 percent of total settlement under the complex conditions．Based on this，the space—time sequence of excavation was optimized in the ways that the excavation method of the reserved cole soil was used along with the stepped CRD method．The sequence was firstly to cut through the upper step is and then to cut through the lower step．The practice has proved that this optimized method is safe and reliable．