In the construction of water transmission tunnels in loess areas, rainfall infiltration greatly affects the stability of the water transmission tunnel envelope. In this paper, the unsaturated seepage theory and numerical simulation method are used to study the deformation and seepage field evolution of the surrounding rock under the coupling effect of extreme rainfall infiltration and excavation disturbance of the tunnel in loess gully area, and the flow-solid coupling analysis is carried out by Plaxis3D model on the mechanical characteristics of surrounding rock and the initial support process during the construction of New Austria method under rainfall infiltration. The results show that the higher the rainfall level, the greater the change of pore water pressure in the surrounding rock, and the more unfavorable to the stability of the surrounding rock; rainfall infiltration and excavation disturbance will accelerate the redistribution process of seepage and stress field in the soil layer; based on the water catchment characteristics of the trench, the water content of the tunnel surrounding rock shows a stepwise increase in the first 5-10 d, and the maximum deformation at the vault, arch waist and arch toe are 17.8 mm, 28.2 mm, 24.6 mm and 21.1 mm, The maximum shear stress is 175 kPa, 182 kPa, 150 kPa, and the deformation rate of the surrounding rock gradually decreases and converges to a specific value about 15~20 d after the end of rainfall, but the stress of the surrounding rock continues to increase, and in order to avoid plastic deformation of the surrounding rock, the secondary lining support can be carried out at 15~20 d. The results of this study can provide reference for the safety control of tunnel construction in loess gully area during extreme rainfall period.