Abstract: It is important for the tunnel portal section to master the interaction mechanism between multi-cavity construction and landslide in asymmetric convex slope section, and adopt effective and reasonable support measures. Based on the Xiaba tunnel, the three-dimensional numerical model with actual slope characteristics is established by FLAC^3D. Combined with the characteristics of field monitoring data, the slope instability mode caused by construction disturbance and the failure characteristics of the tunnel support system is comprehensively analyzed, then the targeted control measure is put forward for the later stage. The results show that : (1) The numerical calculation results are consistent with the actual on-site monitoring. The sliding body mainly slides laterally to the discharge tunnel, with the maximum displacement of 14 cm. The side wall of the discharge tunnel is seriously cracked, and there are obvious signs of cracking in the lining of the right tunnel. (2) After back pressure backfilling and pre-reinforcement of Rock mass to be excavated, the sliding characteristics of the slope are obviously weakened, and the deformation of the lining structure is also effectively controlled. (3) In the strongly weathered basalt layer, the tensile stress at the arch waist and arch foot of the primary support of the main tunnel is relatively large, up to 8.5 MPa; at the junction of strong and medium weathered basalt, there is a tensile stress concentration area. In the later construction, the primary support parameters of the strong weathered basalt layer should be enhanced.