The characteristics of easily being weathered and argillization after meeting with water for the surrounding rock of the Xigeda strata tunnel may seriously affect the rapid and safe construction of multiple tracks of the Chengdu—Kunming railway. Therefore, the study on the deformation characteristic curve and the deformation mechanism of the Xigeda strata tunnel is of great significance. Through field investigation, physical and mechanical test, field monitoring, numerical modeling, and theoretical research, the deformation characteristics of the Xigeda strata tunnel was studied in depth. The results show that the Xigeda strata tunnel mainly suffers from the large deformation of vault, large horizontal convergence, and the uplift of the mud outburst of the inverted arch. Besides, the surrounding rock appears to be the developed strata such as the medium bed, the interbed, and thin stratified horizontal bed. Moreover, the water content greatly influences the mechanical property of the surrounding rock of the Xigeda strata tunnel; and the compressive modulus of the yellow shale sample with interbedded sandstone is the minimum; and the water content has the maximum influences on the cohesive strength of gray shale with interbedded sandstone and on the internal friction angle of French grey shale with interbedded sandstone; through numerical fitting of the deformation characteristic curve under the field monitoring and numerical simulation, the entire deformation process curve of the Xigeda strata tunnel during the excavation can be attained; and at the initial stage of excavation, the deformation develops slowly, and in the mid-term, it turns sharply, and in the middle and later stage, it begins to slow down and in the final stage, it begins to stabilize. Simultaneously, the deformation formulas of surrounding rock of the Xigeda strata under the state of elasticity, plasticity, and the excavation damage zone were derived, and the deformation characteristic curve of the elastoplastic Xigeda strata surrounding rock and the outline of the deformation excavation damage zone of every part of the Xigeda strata tunnel in the four stages. Finally, analysis on the mechanical mechanism of the surrounding rock deformation of the Xigeda strata tunnel was conducted from the perspectives of the constitutive models of mechanics and the argillization of surrounding rock.