Accumulation layer slope has the characteristics of loose structure, strong permeability and easy to be disturbed by construction. Tunnel excavation and rainfall infiltration can lead to the extrusion deformation of the accumulation layer slope body, which causes the deformation or cracking of the tunnel support structure due to the increase of surrounding rock pressure and affects its safety. In this paper, a mountain tunnel in Hunan Province was used as a case study, and the interaction deformation mechanism of the deposit slope and the parallel tunnel under different working conditions was analyzed by numerical method, and the temporal and spatial variation characteristics of the force deformation of the tunnel support structure and the deposit slope were discussed when the tunnel crossed the deposit slope under rainfall infiltration. The calculation results show that rainfall infiltration increases the stress and deformation of the slope and the lining structure of the adjacent parallel tunnel. Moreover, the impact of construction disturbance and rainfall infiltration is most significant at the front edge and slope position of the accumulation layer slope. The stress deformation at the vault of the tunnel lining structure located inside the accumulation layer is larger than that at the arch waist, and the increase in rainfall intensity can significantly enhance this effect. Additionally, rainfall-induced settlement deformation of the tunnel arch crown has a certain time lag.