Abstract: It is important to understand the interaction mechanism between multi-cavity construction and a landslide in an asymmetric convex slope section at the tunnel portal, and to implement effective and reasonable support measures. Based on the Xiaba tunnel, a three-dimensional numerical model incorporating actual slope characteristics was established using FLAC^3D. Combined with the characteristics of field monitoring data, this study comprehensively analyzes the slope instability mode caused by construction disturbance and the failure characteristics of the tunnel support system, and proposes targeted control measure for the later stage. The results show that : (1) The numerical calculation results agree with the actual on-site monitoring data. The sliding body mainly slides laterally toward the discharge tunnel, with a maximum displacement of 14 cm. The side wall of the discharge tunnel shows seriously cracking, and the lining of the right tunnel exhibits obvious crack signs. (2) After implementing backpressure backfilling and pre-reinforcement of the rock mass to be excavated, the sliding characteristics of the slope are significantly weakened, and the deformation of the lining structure is 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 reaches a relatively large value, up to 8.5 MPa; at the junction of strongly and moderately weathered basalt, a tensile stress concentration area forms. Therefore, in the later construction phase, the primary support parameters for the strongly weathered basalt layer should be enhanced.