Abstract: At present, there is no precedent of using the undermining method to construct large-section multi-arch tunnels in fly ash deposits in China, which brings great challenges to the design and construction of the tunnels. Based on the Yanpingba tunnel project, the laboratory similar model tests are carried out to explore the evolution laws of collapse arch and mechanical behaviors of excavation of the double-arch tunnel in fly ash stratum. The results show that the excavation of the multi-arch arch tunnels in the fly ash deposits is highly susceptible to instability, and the failure process is characterized by suddenness, intensity, symmetry and synchronicity. The height of the collapsed arch is about 0.4 times the excavation span of the tunnels. The bearing capacity of the fly ash deposits is seriously insufficient by using the single-side drift method, and the settlements of the left and right vaults are 16.86 cm and 14.91 cm according to the similarity relationship. It is necessary to implement grouting reinforcement for the stratum. The bearing effect of the partition wall can effectively reduce the displacement of the stratum, the shape of stratum settlement is 'bimodal', and the first-excavated tunnel is more affected by excavation than the following one. During the process of conversion from single-tunnel pressure arch to double-tunnel one, the partition wall is easily deflected to the side of the first-excavated tunnel under the action of bias pressure, so it is suggested to add supports to the partition wall in the first-excavated of the existing tunnel.