Based on the Guogaibu tunnel in weathered andesite strata, the characteristics, failure mechanisms, and treatments were investigated. Firstly, the on-site monitoring findings indicated large deformation in the tunnel vault accompanied by the local falling blocks, ultimately resulting in a collapse. Then, laboratory tests were conducted to investigate the failure mechanism. It indicated that the mineral mass of the rock gradually dissolved and microcracks gradually expanded, which was prompted by the combined effects of weathering and groundwater erosion, resulting in a decrease in the strength of andesite, especially in shear strength. Tree factors attributed to collapse include weathering effects, groundwater, and insufficient support. Based on the results, treatment measures such as tunnel circumferential grouting and advanced support were implemented to enhance the overall stability of the tunnel. Finally, on-site monitoring was carried out to investigate the effectiveness of treatments. It revealed that the treatment measures were effective for the stability of the surroundings. The failure analysis method and the implementation of treatment measures can provide guidance for the prevention and control of tunneling in weathered andesite tunnels.