Regarding the safety issues during the excavation process of small radius curved shield tunnels in composite formations of mudstone and backfill soil in Chongqing area, a three-dimensional numerical model was established by Midas. The influencing regularities and sensitivity of curve radius, top thrust force, and unbalanced top thrust force on surface subsidence and surrounding rock deformation were analyzed. The results indicate that: As the radius of the curve increases, the deformation gradually decreases, but the magnitude of the deformation is gentle. There is a critical value for the top thrust, which can cause instability of the palm surface if it is too low, and shear failure of the soil if it is too high. An increase in the proportion of unbalanced thrust will disrupt stress symmetry, and the synergistic effect of soil loss on the weak thrust side and compression on the strong thrust side will exacerbate asymmetric settlement. Sensitivity analysis shows that top thrust plays a dominant role in deformation control, followed by curve radius and unbalanced top thrust. The research results can provide reference for optimizing construction parameters of small radius curved tunnels under similar geological conditions.