Abstract: For the foundation pit consisting of soil and weathered rock stratum, at present, the warning standards of safety risk follow the existing related specifications. They lack pertinence. To more effectively guide the construction of the foundation pit consisting of soil and weathered rock stratum, the failure modes are qualitatively analyzed. Based on an related project, a typical cross section is computed using the three-dimensional finite element method and field monitoring. After synthetic analysis of the monitoring and calculated results, it is pointed out that: (1) the horizontal displacement of retaining structures is a major warning index, and the axial force of inner bracing / soil nailing / anchor bar is auxiliary; (2) the characteristic parameters of warning indexes are the maximum horizontal displacement of retaining structures, the maximum rate, the ratio of the maximum horizontal displacement to excavation depth, the ratio of axial force to earth pressure in the range of excavation depth; (3) the appropriate warning system is established, and compared with the existing related warning index, the design of the above-mentioned foundation pit is too conservative, resulting in high cost of construction, so it is necessary to optimize the design.