Abstract: To explore the deformation mechanisms of tunnel surrounding rock under complex geological conditions and develop appropriate technologies for controlling surrounding rock deformation, this study analyzes the deformation of surrounding rock at the mouth section of a biased tunnel, using the Qingquan Tunnel as a case study. Based on FLAC3D, stability of surrounding rock under different support conditions with and without faults is studied to clarify the deformation mechanism of the surrounding rock and propose effective control measures. The study shows that: (1) excavation disturbances during backward hole excavation under biased conditions cause tensional interaction between the overlying surrounding rock and fault zones, leading to the interlayer rock body bending, rupturing, and stress redistribution, exacerbating the fragmentation of fault zones and resulting in significant surrounding rock deformation. (2) Weak surrounding rock exhibits limited self-stabilization capacity; secondary stress induces sustainable plastic deformation in small clear span tunnel roofs and sidewalls, gradually causing squeezing deformation in support structures over time. Existing support schemes fail to provide sufficient strength and stiffness to resist surrounding rock deformation. (3) Proposed composite control measures of slope surface anchors and deep-buried lateral anti-sliping piles effectively control surrounding rock deformation, mitigate the adverse impact of fractured fault zones on rock stability, and numerical calculation results align closely with on-site monitoring results. The findings provide valuable insights for deformation control of tunnel surrounding rock under similar complex geological conditions.